Fix urllib3.response.HTTPResponse.geturl() return type (#8704)

urllib3.response.HTTPResponse.geturl() return should be `str | None`.

Source: https://github.com/urllib3/urllib3/blob/3951d3cf8f8c8119c455ce50c171dc0ca50cb130/src/urllib3/response.py#L416-L417

stubs/urllib3/urllib3/response.pyi

@@ -99,4 +99,4 @@ class HTTPResponse(io.IOBase):
     def readinto(self, b: bytearray) -> int: ...
     def supports_chunked_reads(self) -> bool: ...
     def read_chunked(self, amt: int | None = ..., decode_content: bool | None = ...) -> Iterator[bytes]: ...
-    def geturl(self) -> bool | str: ...
+    def geturl(self) -> str | None: ...