Test dev Python in GitHub Actions

We also restore 3.10-dev now that PEP 641 was rejected.

.github/workflows/ci.yml

@@ -59,7 +59,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python: ['3.6', '3.7', '3.8', '3.9']
+        python: ['3.6', '3.7', '3.8', '3.9', '3.6-dev', '3.7-dev', '3.8-dev', '3.9-dev']
         check_formatting: ['0']
         extra_name: ['']
         include:
@@ -71,6 +71,12 @@ jobs:
         uses: actions/checkout@v2
       - name: Setup python
         uses: actions/setup-python@v2
+        if: "!endsWith(matrix.python, '-dev')"
+        with:
+          python-version: '${{ matrix.python }}'
+      - name: Setup python (dev)
+        uses: deadsnakes/action@v2.0.2
+        if: endsWith(matrix.python, '-dev')
         with:
           python-version: '${{ matrix.python }}'
       - name: Run tests

.travis.yml

@@ -16,13 +16,6 @@ jobs:
       # https://github.com/pypa/setuptools/issues/2350
       env: SETUPTOOLS_USE_DISTUTILS=stdlib
       dist: bionic
-    - python: 3.6-dev
-    - python: 3.7-dev
-    - python: 3.8-dev
-    - python: 3.9-dev
-    # Temporarily disabled during the high-churn period of 3.10
-    # E.g.: https://github.com/MagicStack/immutables/issues/46
-    #- python: nightly
 
 script:
   - ./ci.sh