Specify the build system in pyproject.toml

[Jackenmen]

What do these changes do?

Adds build-backend to pyproject.toml so that there's no warnings from pip when building from source (+ so that modern setuptools backend is used).

Are there changes in behavior for the user?

pip will no longer show warning:

  WARNING: Missing build requirements in pyproject.toml for multidict==6.0.4 from https://files.pythonhosted.org/packages/4a/15/bd620f7a6eb9aa5112c4ef93e7031bcd071e0611763d8e17706ef8ba65e0/multidict-6.0.4.tar.gz.
  WARNING: The project does not specify a build backend, and pip cannot fall back to setuptools without 'wheel'.

Related issue number

None

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 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.

[Jackenmen]

I'm not sure if a change like this requires a history fragment, let me know :)

[webknjaz]

I think a misc change fragment would be a good idea. It would be weird to make a release without a changelog entry.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (84286ac) 93.65% compared to head (3191e92) 93.65%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #802   +/-   ##
=======================================
  Coverage   93.65%   93.65%           
=======================================
  Files           5        5           
  Lines         504      504           
=======================================
  Hits          472      472           
  Misses         32       32           

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