mypy ignores type checking and produces error for union types when using `is` keyword to check the boolean result from `isinstance()`

Bug Report

When using the is True or is False keywords in the if statement along with isinstance, mypy does not recognize the type checking and produces an error, and ignores the instance type checking.

To Reproduce

from typing import Union

class A:
    prop: str = "Test"
    prop2: int = 999

class B:
    prop: str = "Test 2"

AB = Union[A, B]

if __name__ == '__main__':
    test_var: AB = A()

    if isinstance(test_var, A) is True:
        print(test_var.prop2)```

Playground: https://mypy-play.net/?mypy=latest&python=3.9&gist=7c4401069c861413b72a395a4ce92757

Expected Behavior

I expected that both expressions below were interpreted the same way by mypy as both are syntactically valid and produce the same result:

Using the is keyword to check the result:

    if isinstance(test_var, A) is True:
      pass

No is keyword:

    if isinstance(test_var, A):
      pass

Actual Behavior

main.py:19: error: Item "B" of "Union[A, B]" has no attribute "prop2"  [union-attr]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 0.991
• Mypy command-line flags: --follow-imports silent
• Mypy configuration options from mypy.ini (and other config files):

  •        python_version = 3.9
           warn_return_any = True
           warn_unused_configs = True
           ignore_missing_imports = True
           disallow_untyped_defs = True
           disallow_incomplete_defs = True
           check_untyped_defs = True
           disallow_untyped_decorators = True
           no_implicit_optional = True
           strict_equality = True
           warn_redundant_casts = True
           warn_unused_ignores = True
           show_error_codes = True
    

• Python version used: 3.9

Comienza reproduciendo el ejemplo reportado en el mypy playground enlazado con Python 3.9 y mypy 0.991, comparando isinstance(test_var, A) con isinstance(test_var, A) is True. Rastrea cómo mypy acota los tipos unión para estas condiciones y añade una prueba de regresión; se considera terminado cuando ambas formas reconocen test_var como A sin el error de atributo de unión.