Add flake8-docstrings, fix bunch of flagged issues

[scop]

What do these changes do?

As discussed in #6180 (comment), add flake8-docstrings and fix a bunch of issues flagged by it.

Are there changes in behavior for the user?

No.

Related issue number

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[asvetlov]

Thanks!

[codecov]

Codecov Report

Merging #6276 (217c03b) into master (a3e6a42) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6276   +/-   ##
=======================================
  Coverage   93.30%   93.30%           
=======================================
  Files         103      103           
  Lines       30362    30362           
  Branches     2729     2729           
=======================================
  Hits        28328    28328           
  Misses       1857     1857           
  Partials      177      177           

Flag	Coverage Δ
unit	93.22% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aiohttp/client.py	24.56% <ø> (ø)
aiohttp/client_exceptions.py	96.66% <ø> (ø)
aiohttp/connector.py	93.92% <ø> (ø)
aiohttp/formdata.py	98.93% <ø> (ø)
aiohttp/helpers.py	96.92% <ø> (ø)
aiohttp/http_parser.py	92.45% <ø> (ø)
aiohttp/locks.py	92.85% <ø> (ø)
aiohttp/multipart.py	96.12% <ø> (ø)
aiohttp/pytest_plugin.py	97.48% <ø> (ø)
aiohttp/resolver.py	61.70% <ø> (ø)
... and 9 more

---

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[patchback]

Backport to 3.8: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 47ff9b3 on top of patchback/backports/3.8/47ff9b358bb6147353b1b37b0ac18d641df9869b/pr-6276

Backporting merged PR #6276 into master

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/aio-libs/aiohttp.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/3.8/47ff9b358bb6147353b1b37b0ac18d641df9869b/pr-6276 upstream/3.8

4. Now, cherry-pick PR Add flake8-docstrings, fix bunch of flagged issues #6276 contents into that branch:

   $ git cherry-pick -x 47ff9b358bb6147353b1b37b0ac18d641df9869b

   If it'll yell at you with something like fatal: Commit 47ff9b358bb6147353b1b37b0ac18d641df9869b is a merge but no -m option was given., add -m 1 as follows intead:

   $ git cherry-pick -m1 -x 47ff9b358bb6147353b1b37b0ac18d641df9869b

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Add flake8-docstrings, fix bunch of flagged issues #6276 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.8/47ff9b358bb6147353b1b37b0ac18d641df9869b/pr-6276

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback]

Backport to 3.9: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 47ff9b3 on top of patchback/backports/3.9/47ff9b358bb6147353b1b37b0ac18d641df9869b/pr-6276

Backporting merged PR #6276 into master

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/aio-libs/aiohttp.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/3.9/47ff9b358bb6147353b1b37b0ac18d641df9869b/pr-6276 upstream/3.9

4. Now, cherry-pick PR Add flake8-docstrings, fix bunch of flagged issues #6276 contents into that branch:

   $ git cherry-pick -x 47ff9b358bb6147353b1b37b0ac18d641df9869b

   If it'll yell at you with something like fatal: Commit 47ff9b358bb6147353b1b37b0ac18d641df9869b is a merge but no -m option was given., add -m 1 as follows intead:

   $ git cherry-pick -m1 -x 47ff9b358bb6147353b1b37b0ac18d641df9869b

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Add flake8-docstrings, fix bunch of flagged issues #6276 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.9/47ff9b358bb6147353b1b37b0ac18d641df9869b/pr-6276

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[asvetlov]

Backported by dcffb53 and 46c5df6

The backporting helps to avoid future merge conflicts.