an `UnsafeVariance` modifier

Nobody has claimed this yet.

```py
class A(Generic[T_co]):
def foo(self, t: T_co):
print(t)
```
This code is 100% type safe, the usage of `t` is safe. It would be nice if there was a form of generic where safe usages can be guaranteed, a stop gap would be to introduce an `UnsafeVariance` modifier that just allows these usages.
```py
def foo(self, t: UnsafeVariance[T_co]):
```

# egg
```py
class A[out T]:
def foo(self, t: UnsafeVariance[T]):
reveal_type(t) # object
a = A[int]
a.foo("") # error, expected int, found str
```

Related: #62

Start with the proposed UnsafeVariance examples and read the related issue #62 for existing design context. Trace where generic variance and argument checking are handled in basedmypy, then define completion by supporting the proposed syntax and preserving the shown type-checking behavior. No specific source file or test is named in the issue.

Written by the indexing model from the issue text.