Local variable name is assigned to but never used

https://www.flake8rules.com/rules/F841.html

F401: Module imported but unused

https://www.flake8rules.com/rules/F401.html

Previously running the pep8 target would fail as flake8 was not
installed.

Now install flake8 for the pep8 target.

NOTE: Running the pep8 target fails as there are many warnings/errors.
But it does allow us to run it and possibly work on reducing these
warnings/errors in the future.

In addition, add two checks to the ignore list as black takes care of
formatting. The two checks added to the ignore list are:
  * E501: line too long
  * W503: line break before binary operator

Add an initial mypy test to test gitlab/base.py and gitlab/__init__.py