Bump minimum Python version to 3.9

[janosh]

We could hold off on this. Only reason for doing this is to stay in sync with numpy and the rest of the MP ecosystem (e.g. materialsproject/pymatgen#3283).

[codecov]

Codecov Report

Merging #455 (1f0a570) into main (e062ecc) will not change coverage.
Report is 5 commits behind head on main.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #455   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           20        20           
  Lines         1521      1523    +2     
  Branches       419       419           
=========================================
+ Hits          1521      1523    +2     

Files	Coverage Δ
src/jobflow/core/flow.py	100.00% <100.00%> (ø)
src/jobflow/core/job.py	100.00% <ø> (ø)
src/jobflow/core/reference.py	100.00% <100.00%> (ø)
src/jobflow/core/store.py	100.00% <ø> (ø)
src/jobflow/managers/fireworks.py	100.00% <ø> (ø)
src/jobflow/schemas/job_output_schema.py	100.00% <100.00%> (ø)
src/jobflow/utils/find.py	100.00% <ø> (ø)