Standardize muler dependencies

[gully]

Currently we have dependencies in at least 4 locations:

• setup.py
• environment.yml (technically two of these)
• docs/requirements.txt
• docs/requirements_actions.txt

Having dependencies in multiple places has led to frustration when building documentation and running tests, since it is not clear 1) why there are multiple such files, and 2) which one should be altered or maintained.

@kfkaplan pointed out this problem during #136, and recommends we standardize into a single system of dependencies.

[gully]

This line of code shows where the GitHub Actions are calling the dependencies.

It may be as simple as fixing this line.