Add a patch based on an upstream commit [1].
1: 10707f437f2fc3632067c6a0efa4432f7ebaf362
* gnu/packages/patches/python-gst-fix-build-with-python-3.8.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/gstreamer.scm (python-gst)[source]: Use the patch.