Ensure proper packaging version is used

MrMino · MrMino · commit 7361cdb47ccf · 2021-03-07T22:08:20.000+01:00
packaging.markers.Marker was added in packaging 16.0.
diff --git a/requirements.txt b/requirements.txt
@@ -1,3 +1,3 @@
-packaging
+packaging >= 16.0
 # Pinned pip versions are matched in the GitHub workflows CI matrix.
 pip >= 10.0.1
