[py] fix typing in remote webdriver

The main change is changing the return type hint for
`def get_downloadable_files(self)` from `dict` to `List[str]`,
which is actually what is returned.

Also some cosmetic improvements have been added.

Author: M1troll

py/selenium/webdriver/remote/webdriver.py

@@ -372,6 +372,9 @@ def pin_script(self, script: str, script_key=None) -> ScriptKey:
         """Store common javascript scripts to be executed later by a unique
         hashable ID.
         """
+        if not script:
+            raise ValueError("Script must be a non-empty string")
+
         script_key_instance = ScriptKey(script_key)
         self.pinned_scripts[script_key_instance.id] = script
         return script_key_instance
@@ -582,7 +585,7 @@ def get_cookies(self) -> List[dict]:
         """
         return self.execute(Command.GET_ALL_COOKIES)["value"]
 
-    def get_cookie(self, name) -> Optional[Dict]:
+    def get_cookie(self, name: str) -> Optional[Dict]:
         """Get a single cookie by name. Returns the cookie if found, None if
         not.
 
@@ -591,6 +594,9 @@ def get_cookie(self, name) -> Optional[Dict]:
 
                 driver.get_cookie('my_cookie')
         """
+        if not name:
+            raise ValueError("Cookie name must be a non-empty string")
+
         with contextlib.suppress(NoSuchCookieException):
             return self.execute(Command.GET_COOKIE, {"name": name})["value"]
         return None
@@ -1167,7 +1173,7 @@ def set_user_verified(self, verified: bool) -> None:
         self.execute(Command.SET_USER_VERIFIED, {"authenticatorId": self._authenticator_id, "isUserVerified": verified})
 
     def get_downloadable_files(self) -> List[str]:
-        """Retrieves the downloadable files as a map of file names and their
+        """Retrieves the downloadable files as a list of file names and their
         corresponding URLs.
         """
         if "se:downloadsEnabled" not in self.capabilities: