revert(bzlmod)!: allow bzlmod pip.parse to implicitly use default python version

[chrislovecnm]

Reverts #1303

The main issue is that pip.parse() accepts a locked requirements file -- this means
the requirements are specific to a particular Python version[1]. Because the default Python
version can arbitrarily change, the lock file may not be valid for the Python version
that is used at runtime. The net result is a module will use dependencies for e.g. Python
3.8, but will use 3.9 at runtime. Additionally, the dependencies resolved for 3.8 will
be created under names such as @foo_39 (because that's the python_version pip.parse sees),
which is just more confusing.

BREAKING CHANGE:

• pip.parse() must have python_version explicitly set. Set it to the Python version
  used to resolve the requirements file.

[1] Lock files aren't necessarily version specific, but we don't currently support the
environment markers in lock files to make them cross-python-version compatible.

[rickeylev]

Mostly LGTM, but I want to rewrite the description to give more context. "causes bugs" and "we decided" doesn't give enough information for future readers. It's my EOD now, though, so I don't have the energy to the description and add to the merge queue.