test pins

[delgadom]

There are several ways our pins can be ignored:

• If a conda spec can't be solved with the pins, but can be solved by ignoring pins, it will ignore them
• If pip dependencies conflict with the conda spec, pip will override conda even within the same conda yaml spec
• Later operations can obviously override earlier ones (e.g. notebook trumps base)

To make sure our pins are respected, we should test to make sure the final envs have the requested versions.