psycopg2: correct return type (python#7607)

Fixes the return type of `psycopg2.cursor.fetchone()` to match the psycopg2 code:

https://github.com/psycopg/psycopg2/blob/1d3a89a0bba621dc1cc9b32db6d241bd2da85ad1/psycopg/cursor_type.c#L647-L651
https://github.com/psycopg/psycopg2/blob/1d3a89a0bba621dc1cc9b32db6d241bd2da85ad1/psycopg/cursor_type.c#L748-L786

It also matches the [psycopg2 documentation](https://www.psycopg.org/docs/cursor.html?highlight=copy_from#cursor.fetchone) as well as the [DB-API](https://peps.python.org/pep-0249/#fetchone)

Author: baodrate

stubs/psycopg2/psycopg2/_psycopg.pyi

@@ -368,7 +368,7 @@ class cursor:
     def executemany(self, query: str | bytes | Composable, vars_list: Iterable[_Vars]) -> None: ...
     def fetchall(self) -> list[tuple[Any, ...]]: ...
     def fetchmany(self, size=...) -> list[tuple[Any, ...]]: ...
-    def fetchone(self) -> tuple[Any, ...] | Any: ...
+    def fetchone(self) -> tuple[Any, ...] | None: ...
     def mogrify(self, *args, **kwargs): ...
     def nextset(self): ...
     def scroll(self, value, mode=...): ...