Add py/pl files to $text_paths (remove l/y files?!)

Summary:
I'm guessing this was refactored somewhere down the line and it meant
that Python and Perl files were no longer considered text files.

I'm imagining the old regex was: p(hp|y|l). Therefore I blame CSS.

Test Plan:
Perform an arc lint on a Python or Perl file that has trailing whitespace
on a line. It should prompt you for an autocorrecting lint.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5637