Add Python 3.7 compatibility

[Michael-T-McCann]

Three main changes:

• add math.prod if it doesn't exist
• add typing.Literal if it doesn't exist
• change syntax of parse call in _version.py if it fails

[codecov]

Codecov Report

Merging #301 (66f3b55) into main (4df0046) will decrease coverage by 0.18%.
The diff coverage is 53.33%.

@@            Coverage Diff             @@
##             main     #301      +/-   ##
==========================================
- Coverage   94.31%   94.13%   -0.19%     
==========================================
  Files          53       54       +1     
  Lines        3326     3340      +14     
==========================================
+ Hits         3137     3144       +7     
- Misses        189      196       +7     

Flag	Coverage Δ
unittests	94.13% <53.33%> (-0.19%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
scico/_python37.py	33.33% <33.33%> (ø)
scico/_version.py	82.35% <80.00%> (-2.27%)	⬇️
scico/__init__.py	88.23% <100.00%> (+0.73%)	⬆️

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