Bump mypy to 1.9, add to json.encoder, small fixups (#11549)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

stubs/aiofiles/aiofiles/ospath.pyi

@@ -17,3 +17,5 @@ async def samefile(
     f1: FileDescriptorOrPath, f2: FileDescriptorOrPath, *, loop: AbstractEventLoop | None = ..., executor: Any = ...
 ) -> bool: ...
 async def sameopenfile(fp1: int, fp2: int, *, loop: AbstractEventLoop | None = ..., executor: Any = ...) -> bool: ...
+async def islink(path: FileDescriptorOrPath) -> bool: ...
+async def ismount(path: FileDescriptorOrPath) -> bool: ...