Move optional dependencies out of "dependencies"

[Enkidu93]

Fixes #356. I've confirmed that this is now compatible with silnlp.

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This change is 

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.02%. Comparing base (d496ff6) to head (4c869fe).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #357   +/-   ##
=======================================
  Coverage   92.02%   92.02%           
=======================================
  Files         389      389           
  Lines       24485    24485           
=======================================
  Hits        22532    22532           
  Misses       1953     1953           

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[ddaspit]

@ddaspit reviewed 3 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on pmachapman).

[pmachapman]

@pmachapman reviewed 3 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on Enkidu93).