add type stub for _rsbridge to make PyCharm happy

pylib/anki/_rsbridge.pyi

@@ -0,0 +1,10 @@
+from typing import Any
+
+def buildhash(*args, **kwargs) -> Any: ...
+def open_backend(*args, **kwargs) -> Any: ...
+
+class Backend:
+    @classmethod
+    def __init__(self, *args, **kwargs) -> None: ...
+    def command(self, *args, **kwargs) -> Any: ...
+    def db_command(self, *args, **kwargs) -> Any: ...