ci: build sdist on GitHub Actions

This one-off job is needed to produce the .tar.gz artifact containing
source code.

.github/workflows/sdist.yml

@@ -0,0 +1,22 @@
+on:
+  - push
+  - pull_request
+jobs:
+  sdist:
+    runs-on: 'ubuntu-20.04'
+    steps:
+      - name: Set up Python
+        uses: actions/setup-python@v2
+        with:
+          python-version: '3.9'
+
+      - uses: actions/checkout@v2
+
+      - name: Build sdist
+        run: |
+          python setup.py sdist
+
+      - name: Upload sdist
+        uses: actions/upload-artifact@v2
+        with:
+          path: dist

ci/azure-pipelines.yml

@@ -1,18 +1,4 @@
 jobs:
-- job: Source_Distribution
-  pool:
-    vmImage: ubuntu-18.04
-  steps:
-  - task: UsePythonVersion@0
-    inputs:
-      versionSpec: 3.7
-  - script: |
-        python setup.py sdist
-    displayName: Build
-  - task: PublishPipelineArtifact@1
-    inputs:
-      path: $(System.DefaultWorkingDirectory)/dist
-
 - template: azure-pipelines-anaconda-linux-template.yml
   parameters:
     name: Linux_Anaconda