Highlight numbers in Python import names properly

Summary: This was a typo I made when porting the Pygments lexer.

Test Plan: Made a paste with "import webapp2_extras" and saw that the whole "webapp2_extras" was blue (unlike before, when only "webapp" was).

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10418