Modernize build to use Hatchling

[akx]

This PR switches from setup.py to modern PEP 517 packaging using Hatchling as the build backend.

• The version number is now maintained in dj_database_url/__init__.py (where Hatchling picks it up from). You can use e.g. uvx hatch version minor to do version bumps.
• Also adds a requires-python marker (see discussion in Use typing.Dict type #271).
  • With the marker added, the package actually refuses to install on Python 3.9, so that's removed from the CI matrix.

I made sure to diff the sdist and wheel between the old and new versions, and there are no material differences (as it should be).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7cc1121) to head (06c8054).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #273   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          130       131    +1     
  Branches        14        14           
=========================================
+ Hits           130       131    +1     

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[mattseymour]

I will give this a test today.