Use PyUnicode_IS_ASCII instead of PyUnicode_IS_COMPACT_ASCII, so the following
test can be removed:

   PyUnicode_DATA(op) == (((PyCompactUnicodeObject *)(op))->utf8)

Add also a special case if the result is an empty string.

Thanks to the PEP 3151, select.error (which is just an alias to OSError) has
now an error attribute.

"except" works properly.

More editing is probably desirable.

If maxchar == PyUnicode_MAX_CHAR_VALUE(unicode), we do an useless copy.

 * If needle is 0, try (p[0] >> 16) & 0xff for UCS4
 * Disable fastsearch_memchr_1char() if needle is zero for UCS2 and UCS4

Truncate the buffer size to PY_SSIZE_T_MAX.

Use Py_off_t type (64 bits) instead of off_t (32 bits).