List concatenation involving subclasses (zmypy differs from mypy)

[efroemling]

I stumbled across this case where zmypy gets different results than mypy:

class A:
    pass

class B(A):
    pass

# Mypy/Pyrefly allows this, but zmypy does not:
objs: list[A] = [A()] + [B()]

# Interestingly zmypy infers objs2 to be a list[A] here;
# it only errors if we annotate it as such.
objs2 = [A()] + [B()]
reveal_type(objs2)

% mypy test.py 
test.py:14: note: Revealed type is "builtins.list[test.A]"
Success: no issues found in 1 source file

% zmypy test.py
test.py:9: error: Unsupported operand types for + ("list[A]" and "list[B]")  [operator]
test.py:14: note: Revealed type is "builtins.list[test.A]"
Found 1 errors in 1 file (checked 1 source file)