Add placeholder workflow for building PyO3 wheels. (#1244)

GitHub doesn't allow running new workflows from branches if the workflow
by that name doesn't already exist on the main branch. I'm creating this
placeholder workflow to work around that, so I can then trigger and test
the workflow from my own branch.

.github/workflows/build_langsmith_pyo3_wheels.yml

@@ -0,0 +1,16 @@
+name: Build langsmith_pyo3 wheels
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+  workflow_dispatch:
+
+jobs:
+  hello-world:
+    runs-on: ubuntu-20.04
+    steps:
+      - run: echo 'hello world'