Fix format checking for Unions with alias with custom format

[kreathon]

Motivation

This fix was motivated by the mypy_primer output for https://github.com/apache/spark/ in #11996.

Consider the following code:

Scalar = Union[
    int, float, bool, str, bytes, decimal.Decimal, datetime.date, datetime.datetime, None
]

x: Union[Scalar, "Series"]

'{}'.format(x)  

should lead to

On Python 3 formatting "b'abc'" with "{}" produces "b'abc'", not "abc"; use "{!r}" if this is desired behavior

Implementation

Ensure that the Union is flattened including type aliases before performing the follow checks (for the items):

for a_type in actual_items:
    if custom_special_method(a_type, '__format__'):
        continue
     self.check_placeholder_type(a_type, expected_type, call)
     self.perform_special_format_checks(spec, call, repl, a_type, expected_type)

Test Plan

Add [case testFormatCallFormatTypesUnionAliasWithCustomFormat]

[JelleZijlstra]

I wonder if we should instead suppress this error when the operand is a Union. For other types (e.g. strs) adding !r changes behavior.

[kreathon]

I agree that warnings should not give "unsafe" advises.

Alternatively, the warning could be updated for Union, because, in my opinion, is is really unlikely that the behavior is intended (for Unions).