[PR #6165/850785c3 backport][3.8] Migrate packaging toward declarative setup.cfg

[webknjaz]

This is a backport of PR #6165 as merged into master (850785c).

(cherry picked from commit 850785c)

What do these changes do?

This patch revamps our packaging setup to follow modern practices more closely.

Are there changes in behavior for the user?

No.

Related issue number

N/A

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[lgtm-com]

This pull request fixes 1 alert when merging b280082 into 8a944a7 - view on LGTM.com

fixed alerts:

• 1 for `__eq__` not overridden when adding attributes