Pin Python docker image version to 3.7 to fix build error

[juhoinkinen]

Drone builds were failing. This was due to that the docker base image was pinned to tag 3-slim-buster, whose Python version was updated from 3.7 to 3.8.

Fix by pinning the tag to 3.7-slim-buster.

[codecov]

Codecov Report

Merging #341 into master will increase coverage by 20.25%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #341       +/-   ##
===========================================
+ Coverage   79.25%   99.51%   +20.25%     
===========================================
  Files          55       55               
  Lines        3080     3080               
===========================================
+ Hits         2441     3065      +624     
+ Misses        639       15      -624

Impacted Files	Coverage Δ
annif/analyzer/__init__.py	92.59% <0%> (+3.7%)	⬆️
tests/test_project.py	100% <0%> (+9.35%)	⬆️
annif/backend/__init__.py	100% <0%> (+12.9%)	⬆️
tests/test_analyzer_voikko.py	100% <0%> (+75%)	⬆️
annif/analyzer/voikko.py	100% <0%> (+88.23%)	⬆️
tests/test_backend_fasttext.py	100% <0%> (+88.63%)	⬆️
annif/backend/mixins.py	89.47% <0%> (+89.47%)	⬆️
tests/test_backend_vw_ensemble.py	100% <0%> (+89.74%)	⬆️
annif/backend/vw_ensemble.py	100% <0%> (+89.9%)	⬆️
annif/backend/fasttext.py	98.76% <0%> (+91.35%)	⬆️
... and 3 more

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