fix: Don't require downstream users to register Python toolchains.

The Python toolchains are only needed for dev purposes (doc generation).

Unfortunately, Bazel doesn't provide an easy way to mark a
register_toolchains() call as dev-only. To work around this, we use an
extension to create a repository whose contents depend on if its the
root module or not. If its the root module, it means we're the
rules_testing module and its the dev-dependency case, so write a
build file with the real toolchains. If its not the root module, then
its not the dev-dependency case, so write an empty build file.

Fixes #33