Drop support for Python 3.6 and 3.7 + Upgrade dependencies in GitHub actions

[leifgehrmann]

• Fixed an issue with tox.ini. Evidently in 4.0.6, a breaking change was added that requires commas between environment variables. The error was this: pass_env values cannot contain whitespace, use comma to have multiple values in a single line, invalid values found 'TOXENV CI'
• Python 3.6 and 3.7 are end-of-life, so in the next release of pangocairocffi we will announce that these versions are no longer supported. This includes the following changes:
  • Removing Python 3.6 and 3.7 from setup.cfg.
  • Removing Python 3.6 and 3.7 from the build.yml action.
• In theory we should also support 3.11 and 3.12, so the following changes were made:
  • setup.cfg has been updated to show that the package supports 3.11 and 3.12.
  • Updated the documentation to explicitly say what versions of python we support.
  • coverage.yml and lint.yml actions have been updated to use the latest version of python, 3.12.
• This PR will also update the use of actions/checkout in the GitHub actions to version 4.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b59cfe8) 75.48% compared to head (1b19a32) 75.48%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #31   +/-   ##
=======================================
  Coverage   75.48%   75.48%           
=======================================
  Files           6        6           
  Lines         155      155           
=======================================
  Hits          117      117           
  Misses         38       38           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.