Add missing __eq__ to Mapping

[Dreamsorcerer]

https://github.com/python/cpython/blob/main/Lib/_collections_abc.py#L831

[Dreamsorcerer]

The new error from mypy primer is because they've typed it with Self[K], should be fine for them to type it as object like everything else:
https://github.com/ibis-project/ibis/blob/master/ibis/common/egraph.py#L101

The removed errors from streamlit are exactly the false positives that caused me to look at this (but, in a different library).

[AlexWaygood]

This looks good, just one comment — see above

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

ibis (https://github.com/ibis-project/ibis)
+ ibis/common/egraph.py:101: error: Argument 1 of "__eq__" is incompatible with supertype "Mapping"; supertype defines the argument type as "object"  [override]

streamlit (https://github.com/streamlit/streamlit)
- lib/tests/streamlit/connections/base_connection_test.py: note: In member "test_secrets_property_no_matching_section" of class "ExperimentalBaseConnectionDefaultMethodTests":
- lib/tests/streamlit/connections/base_connection_test.py:58:16: error: Non-overlapping equality check (left operand type: "AttrDict", right operand type: "Dict[<nothing>, <nothing>]")  [comparison-overlap]
- lib/tests/streamlit/connections/base_connection_test.py: note: In member "test_secrets_property_no_secrets" of class "ExperimentalBaseConnectionDefaultMethodTests":
- lib/tests/streamlit/connections/base_connection_test.py:62:16: error: Non-overlapping equality check (left operand type: "AttrDict", right operand type: "Dict[<nothing>, <nothing>]")  [comparison-overlap]

[AlexWaygood]

Thanks!