Use requirements.txt as the single source of truth for Python requirements

[jwodder]

This PR updates the Dockerfile and README to both refer to requirements.txt instead of duplicating the list of requirements.

[satra]

@jwodder - could you please refresh this PR after merging master and running pre-commit?

perhaps add sth like this to the readme:

This repo uses pre-commit for styling and syntax checks. To use in your python + git environment, do:

pip install pre-commit
pre-commit install

After this, it will run the pre-commit checks on every git commit and make any adjustments as necessary, and request that you git commit again.

[jwodder]

@satra Done.

[satra]

thank you @jwodder