This is kind of dirty, but the prospector tool was broken after updating
Python to 3.9, and it seems to no longer be maintained. I forked it to
my personal GitHub account, un-pinned its dependencies, fixed a bug that
came up after updating pylint, and deleted a few dependencies that I
don't use (pylint plugins for Django, Flask, and Celery).

This commit also fixes all the new complaints from the updated pylint,
which were mostly explicitly re-raising exceptions, and some places
where I could use a generator instead of an unnecessary list
comprehension.

This will work for now, but I probably don't want to leave it in this
state. I should probably just stick to using the tools like pylint
directly, since this is now the second time I've needed to replace my
"tool runner" when it stopped being maintained (the first one was
pylama).