Make codecov informational only

Codecov is unable to tell the difference between "the developer removed
lines that had coverage" and "the developer added lines without
coverage". Often maintenance work is the former rather than the latter,
but pull requests still end up being marked as "failed".

Codecov should still submit coverage reports as comments to pull
requests.

.codecov.yml

@@ -4,5 +4,8 @@ coverage:
       default: false
       tests:
         paths: tests
+        informational: true
       two_factor:
         paths: two_factor
+        informational: true
+    patch: off