Improve checks for PEP 604 - alternative Union syntax

[cdce8p]

Is your feature request related to a problem?

Pylint should be able to detect if PEP 604 (alternative Union Syntax) is used inappropriately.
The PEP will be implemented in 3.10, so the following should emit error messages in versions 3 - 3.9:

Alias = str | int  # Should be an error

var: str | int  # Should be an error

def func(arg: int | str):  # Should be an error
    pass

def func2() -> int | str:  # Should be an error
    pass

Each line is a TypeError in Python, eg:

Traceback (most recent call last):
  File "test.py", line 3, in <module>
    Alias = str | int
TypeError: unsupported operand type(s) for |: 'type' and 'type'

With postponed evaluation of annotations enabled (3.7 - 3.9)

from __future__ import annotations

Alias = str | int  # Should be an error

var: str | int

def func(arg: int | str):
    pass

def func2() -> int | str:
    pass

Only the alias definition is evaluated directly and thus an error.

Current situation

No errors are emitted, even in versions prior to 3.10 and without the future import.

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https://www.python.org/dev/peps/pep-0604/