Revise pylint configuration (and linter choice)

[lukpueh]

Description of issue or feature request:
TUF uses a pylint config file that seems to have been copied in verbatim from PyCQA/pylint, listing all defaults explicitly, and adding only a few customizations, which neither fully encode our current style guidelines, nor the style guidelines we plan to adopt (~TBD) for new code.

Note: The same issue exists in securesystemslib, and existed but was fixed in in-toto.

Current behavior:

• pylintrc unnecessarily lists defaults making it hard to read/maintain
• pylintrc does not encode our code style guidelines
• (travis/tox automated testing does not run pylint over test code)

Expected behavior:

1. Decide on style guidelines (Decide on style guideline #1128)
2. Encode chosen guidelines in a new pylintrc-file (see tuf/api: Expose tuf.api as a package (take 2) #1177 for WIP preparatory work)
3. Use new pylintrc for new code including tests
   Note: It is probably not worth fixing the potentially many linter issues in code that might not survive the upcoming refactor.
4. (orthogonal) review and adopt other code quality tools (e.g. @trishankatdatadog has positively mentioned black and lgtm).

[joshuagl]

#1294 suggests our current Tox configuration is not correctly linting the new code (in tuf/api/), even though we have a Pylint statement in Tox and a minimal configuration file for the new style.

[rzr]

tox is linting the code but I noticed that different pylint versions produce different warnings with current rc file.
Maybe some params can be added to prevent this kind of ambiguity.

I'll comment #1294

now back to this ticket, It made sense to align to default configuration, now if there is a need to enforce custom ones , may I suggest that we progressively pick and adapt relevant lines from generated template:
pylint --generate-rcfile > tuf/api/pylintrc

[lukpueh]

Thanks for chiming in, @rzr!

may I suggest that we progressively pick and adapt relevant lines from generated template:
pylint --generate-rcfile > tuf/api/pylintrc

Yes, that's a great idea! Same should go for adopting the rules in the code. See e.g. https://github.com/in-toto/in-toto/pull/296/commits, where I grouped changes by rule and commit, which IMO makes review a lot easier.

Regarding #1294 (comment) ...

Any objection to use black for formatting ?:

No, not at all! :) I have planned to work on this in the near future, but I'd definitely appreciate help.

(cc @fepitre + in-toto/apt-transport-in-toto#36 (comment))

Please note that both revised linter and auto-formatter config should align with our new python style guide.

[lukpueh]

Fixed in #1314 and follow up PRs, we now use a minimally configured pytlint, black and isort for the whole repo (including tests).