Conditional import via try-except renders error for imported type #8319

I'm running python 3.5 (mypy 0.761). TypedDict is not in typing so I have typing_extensions installed (for python < 3.8).

However, the library I'm building supports python up to 3.8, which has TypedDict built-in so I might as well use that.

So I tried conditional import:

try:
    from typing import TypedDict  # type: ignore
except ImportError:
    from typing_extensions import TypedDict

Foo = TypedDict('Foo', {
    'id': str,
})

def whatever(data: Foo) -> None:
    pass

Results in:

typetest.py:12: error: Variable "typetest.Foo" is not valid as a type

Adjusting the import as so makes the problem go away:

import sys
if sys.version_info >= (3, 8):
    from typing import TypedDict
else:
    from typing_extensions import TypedDict

https://mypy.readthedocs.io/en/stable/common_issues.html#python-version-and-system-platform-checks doesn't at least directly suggest to use this style for imports, though. What is the proper/preferred way to do conditional imports?