Allow URLs with paths that end with / as base_url in ClientSession

[Cycloctane]

What do these changes do?

• Allow URLs with paths as base_url in ClientSession.
• Add tests for _build_url to test url joining when base_url includes path.

Are there changes in behavior for the user?

base_url in ClientSession now accepts URLs without paths or with paths that end with /

Is it a substantial burden for the maintainers to support this?

Related issue number

Fixes #6647

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_or_pr_num>.<type>.rst (e.g. 588.bugfix.rst)

  • if you don't have an issue number, change it to the pull request
    number after creating the PR

    • .bugfix: A bug fix for something the maintainers deemed an
      improper undesired behavior that got corrected to match
      pre-agreed expectations.
    • .feature: A new behavior, public APIs. That sort of stuff.
    • .deprecation: A declaration of future API removals and breaking
      changes in behavior.
    • .breaking: When something public is removed in a breaking way.
      Could be deprecated in an earlier release.
    • .doc: Notable updates to the documentation structure or build
      process.
    • .packaging: Notes for downstreams about unobvious side effects
      and tooling. Changes in the test invocation considerations and
      runtime assumptions.
    • .contrib: Stuff that affects the contributor experience. e.g.
      Running tests, building the docs, setting up the development
      environment.
    • .misc: Changes that are hard to assign to any of the above
      categories.

  • Make sure to use full sentences with correct case and punctuation,
    for example:

    Fixed issue with non-ascii contents in doctest text files
    -- by :user:`contributor-gh-handle`.

    Use the past tense or the present tense a non-imperative mood,
    referring to what's changed compared to the last released version
    of this project.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.60%. Comparing base (0a70662) to head (f8b0ed5).
Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9530   +/-   ##
=======================================
  Coverage   98.60%   98.60%           
=======================================
  Files         113      113           
  Lines       35266    35295   +29     
  Branches     4188     4193    +5     
=======================================
+ Hits        34773    34802   +29     
  Misses        331      331           
  Partials      162      162           

Flag	Coverage Δ
CI-GHA	98.48% <100.00%> (+<0.01%)	⬆️
OS-Linux	98.16% <100.00%> (+<0.01%)	⬆️
OS-Windows	96.05% <100.00%> (+<0.01%)	⬆️
OS-macOS	97.37% <100.00%> (+<0.01%)	⬆️
Py-3.10.11	97.24% <100.00%> (+<0.01%)	⬆️
Py-3.10.15	97.64% <100.00%> (+<0.01%)	⬆️
Py-3.11.10	97.71% <100.00%> (-0.01%)	⬇️
Py-3.11.9	97.31% <100.00%> (+<0.01%)	⬆️
Py-3.12.7	98.20% <100.00%> (+<0.01%)	⬆️
Py-3.13.0	98.18% <100.00%> (+<0.01%)	⬆️
Py-3.9.13	97.14% <100.00%> (+<0.01%)	⬆️
Py-3.9.20	97.55% <100.00%> (+<0.01%)	⬆️
Py-pypy7.3.16	97.17% <100.00%> (+<0.01%)	⬆️
VM-macos	97.37% <100.00%> (+<0.01%)	⬆️
VM-ubuntu	98.16% <100.00%> (+<0.01%)	⬆️
VM-windows	96.05% <100.00%> (+<0.01%)	⬆️

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[codspeed-hq]

CodSpeed Performance Report

Merging #9530 will not alter performance

_{Comparing Cycloctane:baseurl_with_path (f8b0ed5) with master (06b2398)}

Summary

✅ 5 untouched benchmarks

[Dreamsorcerer]

Minor suggestions on docs. Otherwise, I think this is ready to merge.

[bdraco]

Formatting of the client reference block isn't great, but its an existing problem so not something that needs to be fixed here

[patchback]

Backport to 3.11: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply f3a3f60 on top of patchback/backports/3.11/f3a3f60b7a4b181272472b188e67d3b5f4ad3306/pr-9530

Backporting merged PR #9530 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/3.11/f3a3f60b7a4b181272472b188e67d3b5f4ad3306/pr-9530 upstream/3.11

4. Now, cherry-pick PR Allow URLs with paths that end with / as base_url in ClientSession #9530 contents into that branch:

   $ git cherry-pick -x f3a3f60b7a4b181272472b188e67d3b5f4ad3306

   If it'll yell at you with something like fatal: Commit f3a3f60b7a4b181272472b188e67d3b5f4ad3306 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x f3a3f60b7a4b181272472b188e67d3b5f4ad3306

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Allow URLs with paths that end with / as base_url in ClientSession #9530 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.11/f3a3f60b7a4b181272472b188e67d3b5f4ad3306/pr-9530

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
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my source is open — https://github.com/sanitizers/patchback-github-app.