Add release workflow.

.github/workflows/release.yml

@@ -0,0 +1,56 @@
+name: Release
+
+on:
+  push:
+    branches:
+      - master
+  release:
+    types:
+      - published
+
+jobs:
+  build:
+    if: github.repository == 'jazzband/contextlib2'
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+
+      - name: Set up Python
+        uses: actions/setup-python@v2
+        with:
+          python-version: 3.8
+
+      - name: Get pip cache dir
+        id: pip-cache
+        run: |
+          echo "::set-output name=dir::$(pip cache dir)"
+
+      - name: Cache
+        uses: actions/cache@v2
+        with:
+          path: ${{ steps.pip-cache.outputs.dir }}
+          key: release-${{ hashFiles('**/setup.py') }}
+          restore-keys: |
+            release-
+
+      - name: Install dependencies
+        run: |
+          python -m pip install -U pip
+          python -m pip install -U setuptools twine wheel
+
+      - name: Build package
+        run: |
+          python setup.py --version
+          python setup.py sdist --format=gztar bdist_wheel
+          twine check dist/*
+
+      - name: Upload packages to Jazzband
+        if: github.event.action == 'published'
+        uses: pypa/gh-action-pypi-publish@master
+        with:
+          user: jazzband
+          password: ${{ secrets.JAZZBAND_RELEASE_KEY }}
+          repository_url: https://jazzband.co/projects/contextlib2/upload