Add Python 3.12

[mahmoud]

Pretty much #283, but without dropping Python 3.7 (and bumping requirements, including newest face release).

[codecov]

Codecov Report

Attention: Patch coverage is 98.05195% with 3 lines in your changes missing coverage. Please review.

Project coverage is 98.10%. Comparing base (2e0c552) to head (8bf19e7).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
glom/core.py	95.65%	2 Missing ⚠️
glom/matching.py	96.77%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #285      +/-   ##
==========================================
+ Coverage   98.05%   98.10%   +0.05%     
==========================================
  Files          27       27              
  Lines        4362     4427      +65     
  Branches      765      631     -134     
==========================================
+ Hits         4277     4343      +66     
- Misses         54       57       +3     
+ Partials       31       27       -4     

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