build: use flit as build backend

[trim21]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.07%. Comparing base (2d53c70) to head (3a33c16).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #515   +/-   ##
=======================================
  Coverage   78.07%   78.07%           
=======================================
  Files          15       15           
  Lines        1587     1587           
=======================================
  Hits         1239     1239           
  Misses        348      348           

Flag	Coverage Δ
3.10	77.50% <ø> (ø)
3.11	77.50% <ø> (ø)
3.12	77.50% <ø> (ø)
3.8	77.27% <ø> (ø)
3.9	77.27% <ø> (ø)
unix-socket	77.65% <ø> (ø)

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