Fix python version lookup error

[evpearson]

Library documentation says that python versions should be provided like this:
python_versions = [
"cp36-cp36m"
"cp38-cp38"
]
This is, in fact, untrue. After 0.2.0, versions needed to be provided like
python_versions = [
"CP36_CP36M"
"CP38_CP38"
]

This PR makes fixes so that the provided documentation is accurate.

Testing:

• I validated that little-cheesemonger 0.2.0 requires python versions be passed in with UPPER_SNAKE_CASE using Wheelhaus
• Added unit tests verifying the different formats

[codecov]

Codecov Report

Merging #82 (1f8d321) into main (6982d8a) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #82   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            8         8           
  Lines          203       203           
  Branches        19        19           
=========================================
  Hits           203       203           

Impacted Files	Coverage Δ
little_cheesemonger/__init__.py	100.00% <100.00%> (ø)
little_cheesemonger/_run.py	100.00% <100.00%> (ø)

[fhightower]

LGTM! Thanks for making this improvement.