tmate session mac

So I can test https://github.com/simonw/datasette/issues/93

.github/workflows/tmate-mac.yml

@@ -0,0 +1,12 @@
+name: tmate session mac
+
+on:
+  workflow_dispatch:
+
+jobs:
+  build:
+    runs-on: macos-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Setup tmate session
+      uses: mxschmitt/action-tmate@v3