refresh miscellaneous repo parts

motivations

we're starting the 2023 update and there are lots of open issues in the repo
small but significant changes to things like github actions and the devcontainer seem worth getting out of the way first
this is also a way to trigger tests and confirm what is/isn't working

replace use of poetry for package management and python script calling with simpler use of requirements.txt and python3 -m ...
use ubuntu-20.04 as the runner for all github actions (using ubuntu-latest has caused unstable action behavior)
refine when tests are triggered
fix failing PUMS download test by normalizing string casing of dataframe columns
add black to python packages and use it on a file as an example of formatting python files
simplify dev container