[2.7] bpo-24960: use pkgutil.get_data in lib2to3 to read pickled grammar files (GH...

[2.7] bpo-24960: use pkgutil.get_data in lib2to3 to read pickled grammar files (GH-4977) (#4980)

This is more complicated than it should be because we need to preserve the
useful mtime-based regeneration feature that lib2to3.pgen2.driver.load_grammar
has. We only look for the pickled grammar file with pkgutil.get_data and only if
the source file does not exist..
(cherry picked from commit 8a5877165e993afb2633cd48da5222326d3f6e0e)