🔧 MAINTAIN: Move from setuptools to flit

[chrisjsewell]

Allows for a PEP 621 compliant pyproject.toml

[codecov]

Codecov Report

Merging #553 (9c03dae) into master (8f53a0b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #553   +/-   ##
=======================================
  Coverage   89.78%   89.78%           
=======================================
  Files          16       16           
  Lines        2104     2104           
=======================================
  Hits         1889     1889           
  Misses        215      215           

Flag	Coverage Δ
pytests	89.78% <ø> (ø)

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Impacted Files	Coverage Δ
myst_parser/__init__.py	88.57% <ø> (ø)

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