Unpin Python micro versions used by stubtest (#7619)

Add new `asyncio` method, fix allowlists

stdlib/asyncio/transports.pyi

@@ -27,6 +27,9 @@ class ReadTransport(BaseTransport):
 class WriteTransport(BaseTransport):
     def set_write_buffer_limits(self, high: int | None = ..., low: int | None = ...) -> None: ...
     def get_write_buffer_size(self) -> int: ...
+    if sys.version_info >= (3, 9):
+        def get_write_buffer_limits(self) -> tuple[int, int]: ...
+
     def write(self, data: Any) -> None: ...
     def writelines(self, list_of_data: list[Any]) -> None: ...
     def write_eof(self) -> None: ...