* Make mappingproxy's nb_or slot symmetric for dict | mp
CPython's mappingproxy_or (Objects/descrobject.c) unwraps a mappingproxy
on either side of '|' to its underlying mapping and delegates back to
PyNumber_Or. This makes 'dict | mp', 'mp | dict', and 'mp | mp' all
produce a plain dict result without dict's own nb_or having to know
about mappingproxy.
RustPython's 'or' slot in 'AsNumber for PyMappingProxy' only handled
the case where the left operand is a mappingproxy. When the slot fired
with a=dict, b=mp (because dict.nb_or returned NotImplemented), it
returned NotImplemented again — so 'dict | mp' raised TypeError. This
propagated to 'UserDict | mappingproxy' (which calls self.data | other).
Rewrite the slot to unwrap a mappingproxy on either side (or both)
before delegating to vm._or on the underlying mappings. The
inplace_or slot is unchanged — '|=' is still rejected.
Unmasks test_userdict.UserDictTest.test_mixed_or.
* Unmask test_types.MappingProxyTests.test_union
The slot fix in this PR also enables this test, which was marked
expectedFailure for the same dict|mappingproxy TypeError. Caught as
UNEXPECTED SUCCESS in CI.
Changjoon