set a 1% theshold for coverage changes

Our tests are somewhat inconsistent in their coverage, so coverage can
go down slightly even with unrelated changes. We also have untested
code, and small tweaks will often be shown as a reduction in coverage.

Setting a 1% threshold will reduce friction on both of these cases. Any
larger changes will still require an increase in coverage, so we should
still trend upward over time.

codecov.yml

@@ -6,3 +6,11 @@ comment:
   require_base: false  # [true :: must have a base report to post]
   require_head: true  # [true :: must have a head report to post]
   hide_project_coverage: false  # [true :: only show coverage on the git diff]
+coverage:
+  status:
+    patch:
+      default:
+        threshold: 1%
+    project:
+      default:
+        threshold: 1%