Experimental speedup patch by Tim Peters (please test!):

It wasn't hard to speed pyclbr by a factor of 3, and I'll attach an
experimental patch for that (experimental because barely tested).  Uncomment
the new "String" stuff and it will deal with strings correctly (pyclbr
currently ignores the possibility), but that slows it down a lot.  Still
faster in the end than current pyclbr, but-- frankly --I'd rather have the
dramatic speedup!