feat(eng-2346): Drop Python 2 support

[chrisimcevoy]

• Drops support for python < 3.8. Going forward we will only support python versions which are not EOL.
• Reworked/simplified requirements declarations:
  • Production requirements are now declared in setup.py.
  • Dev requirements are now declared in requirements.in.
  • We still use pip-compile, but point it at requirements.in and setup.py to generate a requirements.txt, used for local dev and CI.
  • There are no interpreter/version-specific requirements files any more.
• Modernises the (versions of) pre-commit hooks we use.
• Uses a combination of direnv and pyenv to automagically set up dev env.
• direnv now sources a .env file to load the env vars required for pytest suite.
• Removes a bunch of other stuff (e.g. tox.ini) which either isn't used by everyone, or is just in the interest of simplification to get rid of.

[qlty-cloud-legacy]

Code Climate has analyzed commit 25ba2e8 and detected 4 issues on this pull request.

Here's the issue category breakdown:

Category	Count
Duplication	4

The test coverage on the diff in this pull request is 56.8% (50% is the threshold).

This pull request will bring the total coverage in the repository to 59.5% (-0.1% change).

View more on Code Climate.

[mrtam]

🚀 Sweet!