Remove support for Python 3.7 and add testing for 3.12

[MyreMylar]

As title.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (5ee109b) 97.32% compared to head (b853e60) 96.61%.
Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #476      +/-   ##
==========================================
- Coverage   97.32%   96.61%   -0.71%     
==========================================
  Files          83       83              
  Lines       11466    11455      -11     
==========================================
- Hits        11159    11067      -92     
- Misses        307      388      +81     

Files	Coverage Δ
pygame_gui/core/colour_parser.py	86.05% <100.00%> (+0.68%)	⬆️
pygame_gui/core/text/text_box_layout.py	86.23% <ø> (ø)
pygame_gui/core/text/text_line_chunk.py	99.25% <100.00%> (-0.02%)	⬇️

... and 1 file with indirect coverage changes

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