Change default behavior for pickle serialization

[davidmezzetti]

#769 in v7.4.0 modified the behavior of txtai to remove remove Python-specific serialization (i.e. Pickle).

This change will remove the FutureWarning that was introduced and replace it with an error by default when pickle serialization is not directly enabled.

Pickle serialization can still be used with the ALLOW_PICKLE environment variable set to True.