Support only Python 3.9 onwards

[HTenkanen]

Closes #220, closes #215

[codecov-commenter]

Codecov Report

Merging #221 (1fb021d) into master (1d93eb0) will increase coverage by 0.09%.
Report is 9 commits behind head on master.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master     #221      +/-   ##
==========================================
+ Coverage   85.84%   85.94%   +0.09%     
==========================================
  Files          22       22              
  Lines        1173     1167       -6     
  Branches      251      217      -34     
==========================================
- Hits         1007     1003       -4     
+ Misses        115      114       -1     
+ Partials       51       50       -1     

Files	Coverage Δ
pyrosm/boundary.py	61.53% <ø> (ø)
pyrosm/distance.py	100.00% <100.00%> (ø)
pyrosm/pyrosm.py	92.69% <ø> (-0.04%)	⬇️
pyrosm/user_defined.py	100.00% <ø> (ø)
pyrosm/utils/__init__.py	76.29% <100.00%> (ø)
pyrosm/utils/_compat.py	62.50% <ø> (+0.59%)	⬆️
pyrosm/utils/download.py	65.95% <ø> (ø)

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