Update the base testing and notebook environment to python 3.8

[mdeceglie]

Python 3.7 will reach end of life on 2023-06-27 (https://devguide.python.org/#status-of-python-branches)

[kandersolar]

Any reason not to skip ahead to 3.9 or 3.10? I've been using 3.9 as my default for a while now, but even 3.10 is probably old enough to be fine for general use at this point.