Allow None in FilterContext.push type stub

`None` is correctly handled to signify EOF to the buffer, but this
should also be visible in the type stub.

Author: velsinki

av/filter/context.pyi

@@ -10,7 +10,7 @@ class FilterContext:
     ) -> None: ...
     @property
     def graph(self) -> Graph: ...
-    def push(self, frame: Frame) -> None: ...
+    def push(self, frame: Frame | None) -> None: ...
     def pull(self) -> Frame: ...
     def process_command(
         self, cmd: str, arg: str | None = None, res_len: int = 1024, flags: int = 0