freezegun: Fix __exit__ and remove union return types (#4491)

These caused false positives, for example in code like this:

```
    with freeze_time(None) as ft:  # False positive here
        ft.tick(3)  # False positive here
```

third_party/3/freezegun/api.pyi

@@ -1,6 +1,6 @@
 from datetime import date, datetime, timedelta
 from numbers import Real
-from typing import Awaitable, Callable, Iterator, Optional, Sequence, Type, TypeVar, Union, overload
+from typing import Any, Awaitable, Callable, Iterator, Optional, Sequence, Type, TypeVar, Union, overload
 
 _T = TypeVar("_T")
 _Freezable = Union[str, datetime, date, timedelta]
@@ -38,9 +38,9 @@ class _freeze_time:
     def __call__(self, func: Callable[..., Awaitable[_T]]) -> Callable[..., Awaitable[_T]]: ...
     @overload
     def __call__(self, func: Callable[..., _T]) -> Callable[..., _T]: ...
-    def __enter__(self) -> Union[StepTickTimeFactory, TickingDateTimeFactory, FrozenDateTimeFactory]: ...
-    def __exit__(self) -> None: ...
-    def start(self) -> Union[StepTickTimeFactory, TickingDateTimeFactory, FrozenDateTimeFactory]: ...
+    def __enter__(self) -> Any: ...
+    def __exit__(self, *args: Any) -> None: ...
+    def start(self) -> Any: ...
     def stop(self) -> None: ...
     def decorate_class(self, klass: Type[_T]) -> _T: ...
     def decorate_coroutine(self, coroutine: _T) -> _T: ...