Add GitHub actions workflow (#14)

* Add GitHub Actions as CI

Add GitHub Actions workflow that tests the Jupyter notebooks in Python
3.6.8 and Python 3.7.3 Debian Docker images by running an install and
test suite defined in entrypoint.sh

c.f.
https://developer.github.com/actions/managing-workflows/workflow-configuration-options/

* Split workflows for seperate exit statuses

Seperate workflows are entirely different and so will have seperate exit
statuses. If they were not split and one failed then the whole workflow
woull immediatley fail and stop.

c.f. https://jasonet.co/posts/use-github-actions-for-ci/

* use --no-cache-dir as inside Docker images

As this is a CI job with no method to cache anything then there is no
reason to use a cache directory for pip

* Add monthly scheduled job

.github/actions/tests/entrypoint.sh

@@ -0,0 +1,21 @@
+#!/bin/bash
+
+function print_and_run () {
+    printf "\n# %s\n" "${1}"
+    eval $(echo "$1")
+}
+
+print_and_run "which python"
+print_and_run "which pip"
+print_and_run "pip install --upgrade --no-cache-dir -q  pip setuptools wheel"
+print_and_run "pip list"
+
+# Already there, but to make it clear to reader
+print_and_run "cd ${GITHUB_WORKSPACE}"
+
+print_and_run "python --version"
+print_and_run "pip --version"
+print_and_run "pip install --upgrade --no-cache-dir -q -r binder/requirements.txt"
+print_and_run "pip list"
+
+print_and_run "pytest"

.github/main.workflow

@@ -0,0 +1,24 @@
+workflow "Test Python 3.6.8" {
+  on = "push"
+  resolves = ["Python 3.6.8 Debian"]
+}
+
+action "Python 3.6.8 Debian" {
+  uses = "docker://python:3.6.8"
+  runs = "./.github/actions/tests/entrypoint.sh"
+}
+
+workflow "Test Python 3.7.3" {
+  on = "push"
+  resolves = ["Python 3.7.3 Debian"]
+}
+
+action "Python 3.7.3 Debian" {
+  uses = "docker://python:3.7.3"
+  runs = "./.github/actions/tests/entrypoint.sh"
+}
+
+workflow "Monthly Test Python 3.7.3" {
+  on = "schedule(0 0 1 * *)"
+  resolves = ["Python 3.7.3 Debian"]
+}