test: initial Github Actions with Ubuntu & macOS

Running Python standalone tests on multiple OSes would free up Travis CI for tests of various combinations of Node.js and Python as well as tests on other [CPU architectures](https://docs.travis-ci.com/user/multi-cpu-architectures).  __arch: amd64, arm64, ppc64le, s390x__

PR-URL: #1985
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>

Author: cclauss

.github/workflows/Python_tests.yml

@@ -0,0 +1,36 @@
+# TODO: Line 14, enable os: windows-latest
+# TODO: Line 15, enable python-version: 3.5
+# TODO: Line 36, enable pytest --doctest-modules
+
+name: Python_tests
+on: [push, pull_request]
+jobs:
+  Python_tests:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      max-parallel: 15
+      matrix:
+        os: [macos-latest, ubuntu-latest]  # , windows-latest]
+        python-version: [2.7, 3.6, 3.7, 3.8]  # 3.5,
+    steps:
+      - uses: actions/checkout@v1
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v1
+        with:
+          python-version: ${{ matrix.python-version }}
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install flake8 pytest  # -r requirements.txt
+      - name: Lint with flake8
+        if: matrix.os == 'ubuntu-latest'
+        run: |
+          # stop the build if there are Python syntax errors or undefined names
+          flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics
+          # exit-zero treats all errors as warnings. The GitHub editor is 127 chars wide
+          flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
+      - name: Test with pytest
+        run: pytest
+      # - name: Run doctests with pytest
+      #   run: pytest --doctest-modules