simplify & standardize poetry usage with readthedocs

[bmw]

i want this to try to help get automated dependabot PRs working to ideally automate some of that work for us. currently, the inclusion of this requirements.txt file that references a constraints.txt file that is generated on the fly as needed on top of our conventional poetry setup confuses the hell out of dependabot. i've been able to get things working after the changes here on my fork

this PR seems to work on readthedocs