Add a simple test facility with tox and GitHub actions

In order to ensure that feature additions don't break existing Flex
theme deployments, build the docs site on any branch push and PR.

To do so,

* Add a simple tox.ini that runs "pelican -s docs/pelicanconf.py" in
  virtualenvs configured for Python versions 3.6 - 3.9.
* Add an additional GitHub Actions workflow that invokes these
  testenvs with the matching Python version, and runs on any branch push
  and on any pull request.

.github/workflows/pelican-test.yaml

@@ -0,0 +1,28 @@
+name: Pelican test
+on:
+  - push
+  - pull_request
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version:
+          - 3.6
+          - 3.7
+          - 3.8
+          - 3.9
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+      with:
+        submodules: true
+    - name: Set up Python ${{ matrix.python-version }}
+      uses: actions/setup-python@v2
+      with:
+        python-version: ${{ matrix.python-version }}
+    - name: Install dependencies
+      run: |
+        pip install tox tox-gh-actions
+    - name: Test with tox
+      run: tox

tox.ini

@@ -0,0 +1,16 @@
+[tox]
+envlist = py{36,37,38,39}
+skipsdist = True
+
+[gh-actions]
+python =
+    3.6: py36
+    3.7: py37
+    3.8: py38
+    3.9: py39
+    
+[testenv]
+deps =
+  -r docs/requirements.txt
+commands =
+  pelican -s docs/pelicanconf.py