Improve efficiency on CI (#194)

* CI: Add caching for pip packages to save cycles

On subsequent invocations of the test suite on CI, when no dependencies
have been changed, the process can make full use of the package cache,
thus speeding up test runs, and skipping redundant downloads.

When any dependency file changes, the cache will be busted.

* CI: Improve efficiency on GHA

- When pushing to the same branch on a PR, cancel running jobs of the
  same kind.
- Allow running the job manually from the UI, using `workflow_dispatch`.

Author: amotl

.github/workflows/main.yml

@@ -1,6 +1,17 @@
 name: Mocket's CI
 
-on: push
+on:
+
+  # Run on all push events.
+  push:
+
+  # Allow job to be triggered manually.
+  workflow_dispatch:
+
+# Cancel in-progress jobs when pushing to the same branch.
+concurrency:
+  cancel-in-progress: true
+  group: ${{ github.workflow }}-${{ github.ref }}
 
 jobs:
   build:
@@ -16,6 +27,10 @@ jobs:
       uses: actions/setup-python@v4
       with:
         python-version: ${{ matrix.python-version }}
+        cache: 'pip'
+        cache-dependency-path: |
+          Pipfile
+          setup.py
     - name: Install Redis
       run: |
         sudo apt install redis-server