👷 Improve mutation testing caching

[jossmoff]

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Fixes # (issue)

Type of change

Please tick options that are relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[codecov]

Codecov Report

Merging #31 (904b3e6) into main (8f43b82) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #31   +/-   ##
=======================================
  Coverage   93.57%   93.57%           
=======================================
  Files          11       11           
  Lines         420      420           
=======================================
  Hits          393      393           
  Misses         27       27           

Files	Coverage Δ
bookshelf/__init__.py	100.00% <100.00%> (ø)

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