fix: keep Cursor._execmany_returning set until reset

fix: keep Cursor._execmany_returning set until reset

Cursor's _execmany_returning attribute is now initialized at _reset()
and set during _executemany_gen_pipeline(). This way, the attribute is
kept for further results fetch that may occur outside executemany()
context: namely, this is needed because _execmany_returning is used by
_set_results_from_pipeline() which would be called by fetch*() methods.

As a consequence, in _fetch_pipeline(), actual fetch is skipped when
coming from executemany(..., returning=False) as the pgresult would
never be set. This ensures a consistent behavior by raising "no result
available" when calling fetch*() from a non-returning executemany().