Homogenize flake8 line length

[noviluni]

We set a max length of 119 characters and the flake8 on the pipelines was using that value from the pytest.ini (flake8-max-line-length). However, there were some references to 100 characters. I updated the docs (CONTRIBUTING.rst) and the setup.cfg.

On the other hand, when using the flake8 tool directly (instead of using it along with tox), the flake8 file wasn't working. I checked the flake8 documentation, and that file is not accepted, in any case, it should start with a period (.flake8):

Flake8 supports storing its configuration in the following places:

• Your top-level user directory
• In your project in one of setup.cfg, tox.ini, or .flake8.

So I removed it. Then I also realized that it would be a good idea to have a "single point of truth", and as pytest-flake8 will take the values from the setup.cfg, I removed the flake8-max-line-length from the pytest.ini.

Finally, and following with the last idea, I moved the flake8 excusion rules to the setup.cfg.

[codecov]

Codecov Report

Merging #957 (e49c7f0) into master (ad50964) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #957   +/-   ##
=======================================
  Coverage   98.26%   98.26%           
=======================================
  Files         231      231           
  Lines        2597     2597           
=======================================
  Hits         2552     2552           
  Misses         45       45           

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