[lit] Prefer opening files with open (Python 2) rather than io.open which requires io.

[lit] Prefer opening files with open (Python 2) rather than io.open which requires io.

Only rely on Python 3 (io.open) when necessary. This puts TestRunnyer.py closer to how it behaved
before the changes introduced in D43165 and silences a few Windows build bot failures.

Thanks to Stella Stamenova for the patch!

llvm-svn: 329037