Running `pip` twice in same process likely to cause issues

[reynoldsnlp]

The pip docs say...

pip assumes that once it has finished its work, the process will terminate. It doesn’t need to handle the possibility that other code will continue to run after that point, so (for example) calling pip twice in the same process is likely to have issues.

I assume that this is not relevant to our project, since every call to pip is a unique subprocess call, but check with the PyPA maintainers to be sure that this is right.