CLN: MultiIndex._union

[lukemanley]

• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.

No need to special-case missing/NA following the fix in #49442

[phofl]

We have tests covering this?

Looks good otherwise

[lukemanley]

We have tests covering this?

yes, a handful in pandas/tests/indexes/multi/test_setops.py:
test_union_with_missing_values_on_both_sides
test_union_nan_got_duplicated
test_union_keep_ea_dtype_with_na
test_union_with_na_when_constructing_dataframe

[phofl]

cc @lukemanley The Minimum versions failure could be legit, could you have a look? Might be possible that the PerformanceWarning is avoided now

[lukemanley]

cc @lukemanley The Minimum versions failure could be legit, could you have a look? Might be possible that the PerformanceWarning is avoided now

Yep, looks like the PerformanceWarning is now avoided. I've removed the warning check.

[mroeschke]

Thanks @lukemanley