Type detection broken on attribute override and not-equal-assertion

Bug Report

I commonly find myself writing simplified list init parameters. So, if the user provides only a type X, I automatically convert that to list type [X]. In doing so, I found two issue that prevent a proper type detection. See the example code below.

To Reproduce

#!/usr/bin/env python3
from typing import Union, List, Optional

def test(a: Union[List[str], str, None] = None) -> Optional[List[str]]:
    if type(a) == str:
        a = [a]
    assert(type(a) != str)
    return a

Expected Behavior

1. First, the type(a) == str check and consequent list assignment should remove the str type from the Union choices.
2. Second, the assert(!=) check should also remove the str type from the Union options.

Actual Behavior

Both checks do not work and the type is still guessed incorrectly:

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: pip install
• Mypy configuration options from mypy.ini (and other config files): -
• Python version used: 3.8
• Operating system and version: macOS 10.15

Start by running the provided Python reproducer against mypy and inspect the type-narrowing behavior for the type(a) == str assignment and the assert(type(a) != str) check. Done means both branches remove str from the inferred Union and the function is accepted with the expected Optional[List[str]] return type.