Revisions for code coverage (#10)

[blms]

Just realized there were a couple of things I missed in the last PR, for #10:

• Prevent the codecov upload step from running for any python version besides 3.12 (I figure that's our target)
• Set the minimum coverage to 86% for the project, for now

I also noticed there was some Markdown in the README that was messing up on the github render so I converted it to RST syntax.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.50%. Comparing base (81a0579) to head (f3af853).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #16   +/-   ##
=======================================
  Coverage   85.50%   85.50%           
=======================================
  Files          15       15           
  Lines        2215     2215           
=======================================
  Hits         1894     1894           
  Misses        321      321           

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