Add black formatting in tox + rev ignore in git blame

[FernandesMF]

As part of our effort to standardize our project repos, we are adding black formatting to freshmaker in tox.

This commit:

• adds a step with black --check in tox, to verify the code is well formatted
• adds a .git-blame-ignore-revs file to keep track of large reformatting commits that should be ignored by git blame
• adds instructions on how to use .git-blame-ignore-revs automatically in a local environment
• add E203 to flake8 ignores, to prevent conflicts with black's style

For more information, please see the following links:
https://black.readthedocs.io/en/stable/guides/introducing_black_to_your_project.html
https://akrabat.com/ignoring-revisions-with-git-blame/
https://stackoverflow.com/questions/34957237/can-i-configure-git-blame-to-always-ignore-certain-commits-want-to-fix-git-blam/57129540#57129540
https://docs.github.com/en/repositories/working-with-files/using-files/viewing-a-file#ignore-commits-in-the-blame-view

JIRA: CWFHEALTH-2279

"Testing" remarks

I checked that the CI is runnign without errors and that the new black step is being called:
https://github.com/redhat-exd-rebuilds/freshmaker/actions/runs/6252521416/job/16975967623?pr=244#step:4:1929

I also checked that the blame view in github is ignoring the large reformatting commit.

[ElenaKarolinaSemanova]

LGTM 👍

[qixiang]

LGTM, good job.

iiuc, the first commit id in your PR should be preserved after merge, so the .git-blame-ignore-revs should work, but if the first commit id (c3b39ea3d4488c62d4a39c6be5b7ca5fdf6cda84) will be changed, you can open another PR to update it.

(main) A - B - - -D'
             \    |
              C - D

After merge, commit id of D will be changed, but commit id of C should be preserved.