Use `pypi/` instead of `python/` for Python package repos

Created by: olafurpg

Following up from https://sourcegraph.slack.com/archives/C032G1CLM7X/p1652957835148599. The python/ prefix is undesirable because makes it confusing to know the difference between repo:^python and lang:python. With python/ as the prefix it's natural to use the phrase "python repository" to explain such repos but that can quickly be confused any regular repository that has Python code. The "pypi/" prefix makes it clearer that these are python packages only.

For consistency with the npm/ and maven/ prefixes, the pypi/ prefix works better than python/.

cc/ @tjdevries

Test plan

See the CI go green.