Avoid running the workflow twice on PR pushes (aio-libs#700)

When someone force-pushes to a PR or adds a new commit, the workflow would run
_twice_, as both `push` and `pull_request` events would trigger.

The default for `types` is `[opened, reopened, synchronize]`; the latter is the
type that overlaps with `push`.

In addition, existing runs for the same PR are cancelled whenever a push is made.

.github/workflows/ci.yml

@@ -10,10 +10,16 @@ on:
     branches:
       - master
       - ?.?*
+    types: [opened, reopened]
   schedule:
     - cron:  '0 6 * * *'  # Daily 6AM UTC build
 
 
+concurrency:
+  group: ${{ github.ref }}-${{ github.workflow }}
+  cancel-in-progress: true
+
+
 jobs:
 
   lint:

CHANGES/700.misc.rst

@@ -0,0 +1 @@
+Updated the GitHub workflow to avoid running unnecessary extra jobs.