SF patch #438013 Remove 2-byte Py_UCS2 assumptions

Removed all instances of Py_UCS2 from the codebase, and so also (I hope)
the last remaining reliance on the platform having an integral type
with exactly 16 bits.
PyUnicode_DecodeUTF16() and PyUnicode_EncodeUTF16() now read and write
one byte at a time.