When highlighting source, catch "python" for the PHP parser

Summary:
See PHI1065. Normally, Python files are called ".py" and we end up with the lexer coded as "py" by the time we get here, but if you "Highlight As..." and select "Python" we can end up here with "python". (Probably also if you name a file "something.python", although this is very unusual.)

Catch both "py" and "python" for our highlighter.

T3626 should eventually standardize and moot this, but since this patch is only a quarter of a line long it seems reasonable until then.

Test Plan: Added a phlog(), verified we now hit the first-party highlighter with "Highlight As... > Python".

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20231