[CI] add GitHub Actions test workflow

It doesn't look like Travis CI isn't (or was never?) used so let's
instead set up a GitHub Actions workflow to simply run the test suite
across all supported platforms.

This should help keep the codebase in good shape by automatically
testing changes as they're committed and proposed (via PRs). This will
also catch any packaging mistakes as tox does a clean build as part of
the session run. Seemed relevant to mention that :p

.github/workflows/ci.yaml

@@ -0,0 +1,31 @@
+name: CI
+
+permissions:
+  contents: read
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+
+jobs:
+  test:
+    name: Test / ${{ matrix.python }} / ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu, macos, windows]
+        python: ["3.7", "3.8", "3.9", "3.10", "3.11.0-rc - 3.11",
+                  "pypy-3.7", "pypy-3.8", "pypy-3.9"]
+
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Set up Python ${{ matrix.python }}
+        uses: actions/setup-python@v4
+        with:
+          python-version: ${{ matrix.python }}
+
+      - name: Run tests
+        run: pipx run tox -e py -- --verbose

tox.ini

@@ -3,4 +3,4 @@ envlist = py37, py38, py39, py310, py311, pypy3
 isolated_build = True
 
 [testenv]
-commands = python -m unittest
+commands = python -m unittest {posargs}