enable ci-tests and ci-wheels

[bjlittle]

Apologies for this being a bit of a monster 👹

Normally I'd split this up into several different PRs, but we're super keen to make a release of python-stratify and this PR ticks the remaining infra-structure boxes (hopefully)

Essentially, this PR:

• retires cirrus-ci and adopts github actions for CI
• updates the pattern for gha on: branches
• adds the ci-tests gha for testing
• adds the ci-wheels gha for building, testing and publishing sdist and binary wheels, providing support for py39, py310 and py311 on:
  • linux for x86_64 (manylinux2014 PEP599)
  • macosx for x86_64 and arm64
  • win for amd64
• updates the MANIFEST.in in alignment with the pattern adopted on SciTools/iris
• refreshes the README.md
• adds automated test coverage using codecov
• enforces the NEP29 Drop Schedule, thus raising the minimum supported version of python to be >=3.9, and numpy to be >=1.21
• adopts PEP621 to retire our use of setup.cfg and migrate to pyproject.toml
• moves stratify codebase under the src directory
• modernises the setup.py, given PEP518

[HGWright]

Looks great and should be able to merge and release once a couple of minor changes have been made.

[bjlittle]

@HGWright Sweeeeet!

Nice one fella! Thanks 🍻