Test PR comment without base commit coverage, set comment layout

https://docs.codecov.com/docs/pull-request-comments#requiring-the-base-andor-head-commit
https://docs.codecov.com/docs/codecov-yaml#can-i-name-the-file-codecovyml

Author: brianschubert

.github/codecov.yml

@@ -0,0 +1,10 @@
+# Inherits global settings from https://app.codecov.io/account/gh/python/yaml/
+# TODO: enable status checks to fail CI if coverage drops?
+# https://docs.codecov.com/docs/commit-status
+comment:
+  # https://docs.codecov.com/docs/pull-request-comments
+  layout: "condensed_header, diff, flags, files"
+  # TODO: remove; temporary to test PR comment
+  # https://docs.codecov.com/docs/pull-request-comments#requiring-the-base-andor-head-commit
+  require_base: false
+  require_head: false