Add return type to webelement.submit

Without this, `mypy` in strict mode gives me the error:

```
Call to untyped function "submit" in typed context
```

py/selenium/webdriver/remote/webelement.py

@@ -93,7 +93,7 @@ def click(self) -> None:
         """Clicks the element."""
         self._execute(Command.CLICK_ELEMENT)
 
-    def submit(self):
+    def submit(self) -> None:
         """Submits a form."""
         script = (
             "/* submitForm */var form = arguments[0];\n"