Add workflow to publish packages on pypi (#1145)

* Every push on master will cause a release to test pypi
* Every tag on master will cause a real release

Based on https://packaging.python.org/guides/publishing-package-distribution-releases-using-github-actions-ci-cd-workflows/

.github/workflows/publish.yml

@@ -0,0 +1,45 @@
+name: Publish packages
+on:
+  push:
+    branches:
+    - master
+
+jobs:
+  build-n-publish:
+    name: Build release packages
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@master
+    - name: Setup python
+      uses: actions/setup-python@v1
+      with:
+        python-version: 3.9
+
+    - name: Install pypa/build
+      run: >-
+        python -m
+        pip install
+        build
+        --user
+
+    - name: Build a binary wheel and a source tarball
+      run: >-
+        python -m
+        build
+        --sdist
+        --wheel
+        --outdir dist/
+        .
+
+    - name: Publish on test pypi
+      uses: pypa/gh-action-pypi-publish@master
+      with:
+        password: ${{ secrets.TEST_PYPI_API_TOKEN }}
+        repository_url: https://test.pypi.org/legacy/
+
+    - name: Publish release on pypi
+      if: startsWith(github.ref, 'refs/tags')
+      uses: pypa/gh-action-pypi-publish@master
+      with:
+        password: ${{ secrets.PYPI_API_TOKEN }}