🧪 TESTS: Update CI to python 3.7,3.8,3.9

[chrisjsewell]

and also for different lammps versions

[codecov]

Codecov Report

Merging #24 (38b9a9d) into master (5592b68) will decrease coverage by 0.22%.
The diff coverage is n/a.

❗ Current head 38b9a9d differs from pull request most recent head 063fd2e. Consider uploading reports for the commit 063fd2e to get more accurate results

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
- Coverage   77.07%   76.84%   -0.23%     
==========================================
  Files          35       37       +2     
  Lines        2281     2289       +8     
==========================================
+ Hits         1758     1759       +1     
- Misses        523      530       +7     

Flag	Coverage Δ
pytests	76.84% <ø> (-0.23%)	⬇️

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Impacted Files	Coverage Δ
aiida_lammps/utils.py	0.00% <0.00%> (ø)
aiida_lammps/__init__.py	100.00% <0.00%> (ø)

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