TST, CI: turn on codecov patch diffs

* given discussion in numpygh-16405, and activation of
the codecov app instead of webhook, try turning
on patch diffs again to show the coverage for a given
PR

* not sure if this will show any change before the `yml`
is in master; could maybe test on fork initially if needed

.codecov.yml

@@ -8,6 +8,6 @@ coverage:
       default:
         # Require 1% coverage, i.e., always succeed
         target: 1
-    patch: false
+    patch: true
     changes: false
 comment: off