Remove Python 3.9 support until pydantic>=1.7

[CasperWA]

As explained in #728, support for Python 3.9 in the optimade package is essentially invalid until we support pydantic>=1.7.

[codecov]

Codecov Report

Merging #729 (97c02a6) into master (c282454) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #729   +/-   ##
=======================================
  Coverage   92.81%   92.81%           
=======================================
  Files          67       67           
  Lines        3520     3520           
=======================================
  Hits         3267     3267           
  Misses        253      253           

Flag	Coverage Δ
project	92.81% <ø> (ø)
validator	63.72% <ø> (ø)

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[CasperWA]

Can be closed in favor of #731 if wanted.