chore: upgrade docs dependencies and fix warnings

[mcous]

This PR makes sure we're using the latest available mkdocs tools by scoping them to >=3.8, since several libraries involved have dropped 3.7 support. We use 3.11 for the docs build, so this is acceptable.

It also:

• Adds Python 3.12 to the test matrix
• Upgrades black to latest
• Upgrades mypy to latest

[mcous]

Current dependencies on/for this PR:

• main
  • PR #217 👈
  • PR #218
    • PR #219

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[codecov[bot]]

Codecov Report

Merging #217 (b882b01) into main (1dc6abf) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #217   +/-   ##
=======================================
  Coverage   98.36%   98.36%           
=======================================
  Files          19       19           
  Lines         736      736           
  Branches       87       87           
=======================================
  Hits          724      724           
  Misses         10       10           
  Partials        2        2           

Files Changed	Coverage Δ
decoy/__init__.py	100.00% <ø> (ø)
decoy/errors.py	100.00% <ø> (ø)
decoy/matchers.py	89.09% <ø> (ø)
decoy/warnings.py	100.00% <ø> (ø)