[lit] Make lit support config files with .py extension.

Many editors and Python-related diagnostics tools such as
debuggers break or fail in mysterious ways when python files
don't end in .py.  This is especially true on Windows, but
still exists on other platforms.  I don't want to be too heavy
handed in changing everything across the board, but I do want
to at least *allow* lit configs to have .py extensions.  This
patch makes the discovery process first look for a config file
with a .py extension, and if one is not found, then looks for
a config file using the old method.  So for existing users, there
should be no functional change.

Differential Revision: https://reviews.llvm.org/D37838

llvm-svn: 313849