Do not call is_subtype and is_same_type in third pass of semantic analysis

[ilevkivskyi]

We recently eliminated calls to join from semantic analysis and postponed them to callbacks before type checking. However, third pass still calls is_subtype and is_same_type. I think this can cause problems, and it is better to refactor these calls to a later stage (probably also to the callbacks).

This was discovered while working on #3952

[carljm]

We are seeing a crash due to this issue, when (accidentally) using a module in a Union inside a container type. Repro code is:

import datetime
from typing import List, Union

x: List[Union[int, datetime]] = []

x.append(1)

Obviously the code intends to use datetime.datetime instead of datetime, but it should not cause a crash. Remove either the Union or the List and we instead see the expected "invalid type" error.

Posting here because the backtrace passes through https://github.com/python/mypy/blob/master/mypy/subtypes.py#L81 via semantic analyzer pass 3, so this is the root cause. Let me know if you want a dedicated issue for this crash and I will create one.

Not pasting full traceback since it's easily reproducible with the above code sample.

[carljm]

#4384 fixes this crash.