ci: Test on macOS

Test on macOS. Testing on macOS should
allow us to catch C++ ABI issues.

.travis.yml

@@ -6,14 +6,25 @@ dist: focal
 jobs:
   - python: 3.7
   - python: 3.8
+
+  - os: osx
+    osx_image: xcode12.2
+    language: generic
+
 cache: pip
 before_install:
+  - |
+    if [[ $TRAVIS_OS_NAME == "osx" ]]; then
+      python3 -m venv venv
+      source venv/bin/activate
+    fi
   - python --version
 install:
   - pip install "poetry<2,>=1.0" tox
 script:
+  # run style checks on linux, Python 3.8 or higher
   - |
-    if [[ $TRAVIS_PYTHON_VERSION != "3.7" ]]; then
-      tox
+    if [[ $TRAVIS_PYTHON_VERSION != "3.7" && $TRAVIS_OS_NAME != "osx" ]]; then
+      tox || travis_terminate 1
     fi
   - poetry install -v && poetry run pytest -s -v tests