avoid using return t.cast which can prevent attribute access during process teardown

[minrk]

return t.cast(..., X) is equivalent to return X # type:ignore on the result of the expression, but (negligibly) more expensive and can cause errors e.g. preventing access to attributes during __del__ in process teardown, as seen in jupyterhub/repo2docker#1379. It's a good idea to try to avoid anything that may prevent access to attribtues. This means trying our best to keep any module.method calls out of the __get__ path (note: importing from typing import cast is another way to avoid this specific issue and works for methods we really do want to use in this code, but the ignores just seem better than the casts to me, anyway).

Note this is only applied to return statements, since the type narrowing of t.cast necessarily has no effect there other than explicitly ignoring a single type check. Other uses of t.cast in the middle of some code for type narrowing are still totally sensible.