Add a “derivations” extra

[musicinmybrain]

Use an extra to declare optional dependencies (currently, sympy) that are only used for the derivations.

Currently, sympy is listed only as a test dependency in test-requirements.txt, which isn’t completely accurate, since it’s used throughout transforms3d.derivations. However, it’s a fairly heavy-weight dependency, and isn’t needed at all for users that don’t need to import from transforms3d.derivations. Adding the extra means that users who do plan to use derivations can depend on transforms3d[derivations] rather than having to add a direct dependency on sympy.

In the python-transforms3d package in Fedora, we currently deal with the use of sympy in the derivations by adding a weak dependency on sympy to the python3-transforms3d package, but these weak dependencies still installed by default – and sympy has its own weak dependency on a LaTeX environment, so all of TeXLive is pulled in. With a derivations extra, we could drop the weak dependency and offer a python3-transforms3d+derivations metapackage instead.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.87%. Comparing base (d6bb759) to head (a242f8c).
Report is 8 commits behind head on main.

Additional details and impacted files

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##             main      #64      +/-   ##
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- Coverage   97.00%   96.87%   -0.14%     
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  Files          27       27              
  Lines        1471     1471              
  Branches      140      140              
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- Hits         1427     1425       -2     
- Misses         31       33       +2     
  Partials       13       13              

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