Fix string/unicode buffer size parsing

Fix string/unicode buffer size parsing

It seems that CPython allows passing "wrong" pointers for et#
(Py_ssize_t instead of int), but PyPy has problems with this, and
wrongly casts the pointers, leading to unsigned/signed conversion
failures.

This bug was introduced in commits 433c5852 and 8aa6fd81; I'm no
longer sure why bufsize was changed from int to Py_ssize_t as well,
but passing an int value to a C function expecting an ssize_t should
be safe as the compiler should do the required conversions behind the
scenes (as long as it has correct information about the types).

Closes gh-1.