Always use python -m with pip

[jhkennedy]

I noticed some inconsistencies around how pip is invoked in the package guide, so I've gone through and made sure all pip commands are invoked with python -m pip ... as recommended in the guide here:
https://www.pyopensci.org/python-package-guide/package-structure-code/declare-dependencies.html#install-dependency-groups

[lwasser]

Thank you so much for this @jhkennedy !! merging it now!