Rename master to main

[hugovk]

Plus:

• Use python-build instead of calling setup.py directly: https://blog.ganssle.io/articles/2021/10/setup-py-deprecated.html
• A few other consistency things

[codecov]

Codecov Report

Merging #43 (6bd010f) into main (232c9bc) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #43   +/-   ##
=======================================
  Coverage   55.91%   55.91%           
=======================================
  Files           4        4           
  Lines         431      431           
=======================================
  Hits          241      241           
  Misses        190      190           

Flag	Coverage Δ
GHA_Ubuntu	55.91% <ø> (ø)
GHA_macOS	55.91% <ø> (ø)

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