don't report constrained `TypeVar` with variance if the constraints are assignable

```py
T1 = TypeVar("T1", object, int, covariant=True) # expect no error
T2 = TypeVar("T2", int, str, covariant=True) # expect error

class A(Generic[T1]):
def get(self) -> T: ...

a_int = A[int]()
a_obj = a_int # valid assignment
```

Start by locating the diagnostic that reports variance on constrained TypeVars and trace how constraint assignability is checked. Add regression coverage for the two TypeVar examples: the object/int constraints should produce no error, while the int/str constraints should still produce an error.

Written by the indexing model from the issue text.