Add pip as an explicit dependency

[spencermathews]

Creating the earthml conda enviroment according to the installation instructions leads to a warning related to pip:

$ 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.

Including pip in the environment prevents this warning, avoids any ambiguity surrounding pip dependencies, and conforms to the best practices as I interpret them.

[jbednar]

Thanks! I think this should be merged, but it looks like the environment needs to be fixed first, so that tests will pass again. @jsignell may be able to get to that next week...

[spencermathews]

What's up with those checks anyway? And why should they fail due to such an innocuous change?

[jbednar]

It's presumably due to one of our dependencies having released a new and slightly incompatible version; nothing to do with your change. Any new PR opened now will probably show the same problem...