Type variables in Callables don't unify

Issue type: Bug
Python version: 3.8
mypy version: 0.760
mypy flags: whatever the defaults are on https://mypy-play.net/

I discovered this issue when playing around with a monad-y Either thing (and here's it working in Typescript)

Here is (I think) a minimal repro:

from typing import Callable, TypeVar, Generic

def foo() -> int:
    return 1

T = TypeVar('T')
bar: Callable[[], T] = foo

main.py:7: error: Incompatible types in assignment (expression has type "Callable[[], int]", variable has type "Callable[[], T]")

I would expect that in the type declaration of bar, T would unify with int, but clearly it does not.

Start with the minimal reproduction in main.py and run it using the mypy version and default flags described in the issue. Compare the Callable assignment result with the expected TypeVar unification, then trace the relevant type-inference behavior in mypy. Done means the reproduction is accepted with the intended inferred type and regression coverage is added.