Refresh support matrix for Python 3.12

[maxbelanger]

• CI now tests Python 3.9, 3.10, 3.11, 3.12, we no longer test 3.5, 3.6, 3.7.
• We no longer advertise support for Python 2.7. This is inevitable as GitHub action runners no longer support Python 2.7, so we can no longer run CI for this version.
• We refer to runners with an explicit version (e.g. macos-13 , no more *-latest). This previously broke most test configurations when macOS defaulted to 14, which is Apple Silicon.
• Add setuptools to install_requires, since on Python 3.12+ this is no longer built-in. The library depends on this at runtime (pkg_resources).

Supersedes #481 and #484. These changes will require a major version bump.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.56%. Comparing base (487793d) to head (711cacc).

❗ Current head 711cacc differs from pull request most recent head a5912f6. Consider uploading reports for the commit a5912f6 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #490      +/-   ##
==========================================
+ Coverage   63.69%   64.56%   +0.86%     
==========================================
  Files          31       31              
  Lines       52609    53896    +1287     
  Branches     3841     3841              
==========================================
+ Hits        33510    34798    +1288     
+ Misses      19064    19063       -1     
  Partials       35       35              

Flag	Coverage Δ
unit	64.56% <ø> (+0.86%)	⬆️

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[maxbelanger]

Note that integration tests are green save for one (malformed token handling). That seems to be a backend change that should be investigated separately.