Add pip to environment.yml

[jacob-rosenthal]

Pip is currently not listed in environment.yml file.
Conda gives the following warning:

(base) jupyter@rosenthal-dxvm:~/pathml$ conda env create -f environment.yml

Warning: you have pip-installed dependencies in your environment file, but you do not list pip 
itself as one of your conda dependencies. Conda may not use the correct pip to install your 
packages, and they may end up in the wrong place.  Please add an explicit pip dependency. 
I'm adding one for you, but still nagging you.