Switch to Conda for readthedocs builds

[michaelweinold]

Compare readthedocs/readthedocs.org#857: currently, conda is not supported for builds, making an additional pip requirements file necessary.

[michaelweinold]

Currently, the requirements file is setup/pip_requirements.txt, which is referenced in the readthedocs yaml .readthedocs.yaml.

[michaelweinold]

N̶o̶ ̶a̶d̶d̶e̶d̶ ̶b̶e̶n̶e̶f̶i̶t̶ ̶-̶ ̶t̶h̶e̶ ̶̶p̶i̶p̶̶ ̶i̶n̶s̶t̶a̶l̶l̶ ̶f̶r̶o̶m̶ ̶̶s̶e̶t̶u̶p̶/̶p̶i̶p̶_̶r̶e̶q̶u̶i̶r̶e̶m̶e̶n̶t̶s̶.̶t̶x̶t̶̶ ̶w̶o̶r̶k̶s̶ ̶j̶u̶s̶t̶ ̶f̶i̶n̶e̶.̶

[michaelweinold]

Issues arise from different versions of graphviz available through PyPi and conda-forge. Best to use ONE environment file for both local and remote builds. Switched to using a conda environment at readthedocs.org with 750fbb6