Replace setup.py with pyproject.toml, update CI

[chrisjonesBSU]

This uses a pyproject.toml file which is recommended over setup.py now.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.90%. Comparing base (a600b73) to head (2331b31).
Report is 53 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #182      +/-   ##
==========================================
+ Coverage   94.77%   94.90%   +0.13%     
==========================================
  Files          26       26              
  Lines        1970     2022      +52     
==========================================
+ Hits         1867     1919      +52     
  Misses        103      103              

Files with missing lines	Coverage Δ
flowermd/__init__.py	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes