Only run CI on changes that would affect its outcome

There are two ways to do this¹: paths and paths-ignore. I chose paths
since it is more self-documenting.

Since this specified on the workflow level and applies to all jobs, in
the future we can consider splitting this file into multiple - one file
per group of jobs that depends on the same set of files (e.g. the docs
project is independent of other testing, so it's useless to run on
changes to test files).

¹ <https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#running-your-workflow-only-when-a-push-affects-specific-files>

.github/workflows/ci.yaml

@@ -3,6 +3,21 @@ on:
   push:
     branches:
       - master
+    paths:
+      - .github/workflows/ci.yaml
+      - augur/
+      - docs/
+
+      # Used for testing
+      - bin/
+      - scripts/
+      - tests/
+      - .coveragerc
+      - .cramrc
+      - .flake8
+      - codecov.yml
+      - mypy.ini
+      - pytest.python3.ini
 
   # Routinely check that tests pass with new versions of dependencies.
   schedule: