Make Jupyter notebooks use the fast builtin Python highlighter

Summary:
Ref T13105. This is silly, but "py" and "python" end up in different places today, and "py" is ~100x faster than "python".

See also T3626 for longer-term plans on this.

Test Plan: Reloaded a Jupyter notebook, saw it render almost instantly instead of taking a few seconds.

Reviewers: mydeveloperday

Reviewed By: mydeveloperday

Maniphest Tasks: T13105

Differential Revision: https://secure.phabricator.com/D19273