Improve CI reproducibility and caching

[siddharth-krishna]

• We should pin all dependency versions in requirements.txt and use that in the CI, so that all CI runs use the exact same versions (unless/until we manually bump up a version)
• We should also take a hash of the requirements.txt file and include it in the cache key, since the cache is dependent on the version of hashlib used

Originally posted by @siddharth-krishna in #38 (review)