Add return type for session close (#305)

Co-authored-by: Tadas Gedgaudas <[email protected]>

curl_cffi/requests/session.py

@@ -691,7 +691,7 @@ def __enter__(self):
     def __exit__(self, *args):
         self.close()
 
-    def close(self):
+    def close(self) -> None:
         """Close the session."""
         self._closed = True
         self.curl.close()
@@ -985,7 +985,7 @@ async def __aexit__(self, *args):
         await self.close()
         return None
 
-    async def close(self):
+    async def close(self) -> None:
         """Close the session."""
         await self.acurl.close()
         self._closed = True