Python 3 - Port use of string.maketrans and don't use sets.Set.

`sets.Set` has been deprecated in favor of `set` since 2.6, and
`string.maketrans` has to be special cased.  In Python 3 there
is `str.maketrans`, `bytes.maketrans`, and `bytearray.maketrans`
and you have to choose the correct one.  So we need to introduce
a runtime version check at this site.

llvm-svn: 252348