Upgrade tooling, including dealing w/ Python 3.7 EOL

[florimondmanca]

Our yearly tooling and packaging refresher!

This PR gets this package up to date with the latest packaging and tooling practices...

• Remove flake8 and isort in favor of ruff
• Use pyproject.toml
• Use build (PEP 517)
• Use latest CI template, update Makefile accordingly

Also does the following:

• Drop 3.7 support, as it is EOL since June 2023 (and e.g. latest wheel can't be installed on it)
• Test on Python 3.11
• Update dev dependencies, deal with django-test-migrations not supporting Django < 3.2
• Update guide.md installation instructions to be in line with README.md
• Use Postgres 15 on CI (11 is outdated)

[florimondmanca]

This is a leftover from #244 which I didn't see before.

We don't need this test anymore now that we always use SHA512.

[codecov-commenter]

Codecov Report

Merging #247 (b37ca59) into master (f707b0c) will not change coverage.
The diff coverage is 100.00%.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

@@            Coverage Diff            @@
##            master      #247   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           25        24    -1     
  Lines          629       622    -7     
=========================================
- Hits           629       622    -7     

Files Changed	Coverage Δ
src/rest_framework_api_key/crypto.py	100.00% <ø> (ø)
.../rest_framework_api_key/migrations/0001_initial.py	100.00% <ø> (ø)
...work_api_key/migrations/0002_auto_20190529_2243.py	100.00% <ø> (ø)
...work_api_key/migrations/0003_auto_20190623_1952.py	100.00% <ø> (ø)
...work_api_key/migrations/0005_auto_20220110_1102.py	100.00% <ø> (ø)
tests/test_permissions.py	100.00% <ø> (ø)
src/rest_framework_api_key/__init__.py	100.00% <100.00%> (ø)
...ework_api_key/migrations/0004_prefix_hashed_key.py	100.00% <100.00%> (ø)
tests/test_migrations.py	100.00% <100.00%> (ø)

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