Replace typing-extensions Literal with the type of the Literal

[pietermarsman]

Pull request

Fixes #676

• Removes typing-extensions Literal and replaces it with the type of the Literal.
• Changes return type of open_filename contextmanager __exit__ method from False to None.
• Split tox.ini environments into 4 parts. One for mypy, flake8, nosetests and sphinx.This should fail for the first commit.

How Has This Been Tested?

Made the cicd fail when changing tox.ini. Then fixed it by removing the typing-extensions.

Checklist

• I have added tests that prove my fix is effective or that my feature
  works
• I have added docstrings to newly created methods and classes
• I have optimized the code at least one time after creating the initial
  version
• I have updated the README.md or I am verified that this
  is not necessary
• I have updated the readthedocs documentation or I
  verified that this is not necessary
• I have added a consice human-readable description of the change to
  CHANGELOG.md

[0xabu]

@pietermarsman sorry for the mess. This PR looks way more complex than I anticipated. CI issues aside, is there a reason we can't just add typing-extensions to our runtime dependencies in setup.py?

[0xabu]

is there a reason we can't just add typing-extensions to our runtime dependencies in setup.py?

See #678 for what I mean.

[pietermarsman]

Yep, I understand.

Not sure yet. I like to keep dependencies minimal but this one seems ok. Let's discuss in gitter.

I wanted to improve the cicd in any case such that it will raise errors in the future.

[0xabu]

Agreed on the CI improvements.

As for the dependency -- no big deal either way. There's only one non-trivial use of Literal in some questionable code in pdf2txt, and I hope to send you a bigger PR that cleans it up anyway.

(Hint: #657 fixes a related open issue there.)

[htInEdin]

For the actual fix to psparser and pdf2txt, I prefer e.g.
Union[float, typing.Literal("disabled") if typing.hasattr('Literal') else str]
because it's future-proof.

[pietermarsman]

@0xabu I'm not going to include the typing-extensions for now. The __exit__ typing is fixed by returning None (returning False makes only sense if returning True is also an option, which wasn't). And then there is just one use case left for the typing-extensions package.

Don't hesitate to add it in the future in another PR, if you have additional use cases for it. I just think that having one usage of it is not enough.