Update our classifiers, better licensing, and migrate to hatch

[agriyakhetarpal]

Description

• Remove discouraged license classifier and switch to PEP 639 licensing standards (SPDX license expression and license-file)
• Switch the build backend to hatchling instead of setuptools (faster, less buggy, better-licensed)
• Move our project from alpha to a beta stage

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #).

Important checks:

Please confirm the following before marking the PR as ready for review:

• No style issues: $ pre-commit run or $ nox -s pre-commit (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: nox -s tests
• The documentation builds: nox -s doctest
• Code is commented for hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[agriyakhetarpal]

I'm not actually sure why we have multiprocessing_bench.py in the root of our repository – perhaps it was meant to be stored elsewhere?

[NicolaCourtier]

I checked and we decided it can be removed: #916

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.88%. Comparing base (4628a65) to head (5e5a709).
⚠️ Report is 51 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #941      +/-   ##
===========================================
+ Coverage    89.41%   90.88%   +1.47%     
===========================================
  Files           63       73      +10     
  Lines         4857     5686     +829     
===========================================
+ Hits          4343     5168     +825     
- Misses         514      518       +4     

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[NicolaCourtier]

Looks good to me, thanks @agriyakhetarpal !