testcase: Drop __enter__() return types

Most recent ruff complains about

> PYI034 `__enter__` methods in classes like `SpawnedMock` usually return `self` at runtime
> help: Use `Self` as return type

But neither `Self` nor `self` nor `"Self"` actually works. Let's not
bother, mypy figures out the return type just fine.

Author: martinpitt

dbusmock/testcase.py

@@ -135,7 +135,7 @@ def __init__(self, bustype: BusType):
     """
         )
 
-    def __enter__(self) -> "PrivateDBus":
+    def __enter__(self):
         # Allow for start() to be called manually even before the `with`
         if self._daemon is None:
             self.start()
@@ -439,7 +439,7 @@ def obj(self):
         """The D-Bus object this server was spawned for"""
         return self._obj
 
-    def __enter__(self) -> "SpawnedMock":
+    def __enter__(self):
         return self
 
     def __exit__(self, exc_type, exc_val, exc_tb):