Add config tests

[murilo-cunha]

As suggested in #18, add more granular tests for _config_callback.
Make minor improvements by parsing a sample pyproject.toml file.

[codecov]

Codecov Report

Merging #23 (b84abe9) into main (9f58381) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #23      +/-   ##
==========================================
+ Coverage   95.70%   95.73%   +0.02%     
==========================================
  Files          19       19              
  Lines         768      773       +5     
==========================================
+ Hits          735      740       +5     
  Misses         33       33              

Impacted Files	Coverage Δ
databooks/config.py	100.00% <ø> (ø)
tests/test_cli.py	100.00% <100.00%> (ø)
tests/test_data_models/test_notebook.py	100.00% <100.00%> (ø)