Feature/add py37 support

[murilo-cunha]

As per suggested in #20, add python 3.7 support! 🎉

• Change strategy for version bumping (don't use importlib.metadata)
• Add typing-extensions to use Protocol and runtime_checkable
• Change CI to bump version, add 3.7 in testing matrix, etc.

[codecov]

Codecov Report

Merging #22 (8379da7) into main (cd6ebbf) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
- Coverage   95.70%   95.70%   -0.01%     
==========================================
  Files          18       19       +1     
  Lines         769      768       -1     
==========================================
- Hits          736      735       -1     
  Misses         33       33              

Impacted Files	Coverage Δ
databooks/cli.py	93.05% <100.00%> (-0.28%)	⬇️
databooks/data_models/base.py	88.13% <100.00%> (+0.20%)	⬆️
databooks/version.py	100.00% <100.00%> (ø)
tests/test_cli.py	100.00% <100.00%> (ø)
tests/test_data_models/test_notebook.py	100.00% <100.00%> (ø)

[devdnhee]

installed your feature branch on my poetry env accepting 3.7 and no dependency issues, and did a few databooks commands, worked like a charm!