chore: merge constraints.txt and setup.cfg into pyproject.toml to simplify setup

[ThosRTanner]

Fixes #786

Why is this needed?

Currently constraints are specified in 3 places which makes it hard to track what you need to change. I'm not an out and out lover of pyproject.toml but having all module setup in one place makes it easier to control package version dependencies.

All that is left in setup.cfg is the tag_build = dev to tag dev builds as such, as I've been unable to find an obvious equivalent in pyproject.toml

Proposed Changes

• Move constraints.txt and setup.cfg info into pyproject.toml. This also requires us to:
  • Update setuptools to a version that supports dependency groups
  • Update the pytest version for python3.8 to the latest version that still supports python3.8
  • Update the required version of tox
• Differentiate between the project's optional dependencies and the internal tooling dependencies
• Update tox.ini to use new dependency groups
• Remove the top level Makefile which has not been maintained and refers to non existent requirements file
• Move control of sphinx dependencies into docs/requirements.txt
  • also make tox -e docs actually generate the docs.

Does this PR introduce any breaking change?

The internal dependencies are no longer leaked from setup.cfg, and the top level Makefile has been removed. This shouldn't affect anyone, but one never knows...

Codecov report

This appears to be because it's two commits out of date. This change hasn't affected what code is covered during tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.41%. Comparing base (e894342) to head (c24cafe).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #798      +/-   ##
==========================================
- Coverage   96.65%   95.41%   -1.24%     
==========================================
  Files          27       27              
  Lines        3554     3858     +304     
==========================================
+ Hits         3435     3681     +246     
- Misses        119      177      +58     

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