Add workflow to publish releases to PyPI.

Fixes #73

.github/workflows/publish-to-pypi.yaml

@@ -0,0 +1,36 @@
+name: Build, and perhaps publish to PyPI
+
+on: push
+
+jobs:
+  build-and-publish:
+    name: Build and optionally publish
+    runs-on: ubuntu-latest
+    environment:
+      name: pypi
+      url: https://pypi.org/p/pygtfs
+    permissions:
+      id-token: write
+    steps:
+    - uses: actions/checkout@v3
+    - name: Set up Python
+      uses: actions/setup-python@v4
+      with:
+        python-version: "3.x"
+    - name: Install pypa/build
+      run: >-
+        python3 -m
+        pip install
+        build
+        --user
+    - name: Build a binary wheel and a source tarball
+      run: >-
+        python3 -m
+        build
+        --sdist
+        --wheel
+        --outdir dist/
+        .
+    - name: Publish tagged versions to PyPI
+      if: startsWith(github.ref, 'refs/tags')
+      uses: pypa/gh-action-pypi-publish@release/v1