ovsdb-idl.at: Return stream open_block python tests.

ovsdb-idl.at: Return stream open_block python tests.

Invocations of CHECK_STREAM_OPEN_BLOCK_PY was accidentally removed
during python2 to python3 conversion.  So, these tests was not
checked since that time.

This change returns tests back.  CHECK_STREAM_OPEN_BLOCK_PY needed
updates, so instead I refactored function for C tests to be able to
perform python tests too.  Also, added test for python with IPv6.

Fixes: 1ca0323e7c29 ("Require Python 3 and remove support for Python 2.")
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Acked-by: Gaetan Rivet <grive@u256.net>