CI: Add nightly upload of wheels to Anaconda Cloud

Add nightlies.yml GitHub Action workflow that uses the gh CLI API to
download the 'wheels' artifact from the latest completed build of the
cibuildwheel workflow from the 'main' branch. This is done by getting
the fields 'event', 'status', and 'databaseId' from the output of
`gh run list` as JSON and then using jq to filter for 'push' events
(which correspond to merged PRs to the 'main' branch) that have
completed runs. This is then sorted by the databaseIds in descending
order to get the latest completed build run number. Then the 'wheels'
artifact can be downloaded.

The downloaded wheels are then uploaded to the scipy-wheels-nightly
Anaconda Cloud organization (https://anaconda.org/scipy-wheels-nightly)
using the anaconda-client CLI API.

Author: matthewfeickert

.github/workflows/nightlies.yml

@@ -0,0 +1,64 @@
+name: Upload nightly wheels to Anaconda Cloud
+
+on:
+  # Run daily at 1:23 UTC to upload nightly wheels to Anaconda Cloud
+  schedule:
+    - cron: '23 1 * * *'
+  # Run on demand with workflow dispatch
+  workflow_dispatch:
+
+jobs:
+  upload_nightly_wheels:
+    name: Upload nightly wheels to Anaconda Cloud
+    runs-on: ubuntu-latest
+    if: github.repository_owner == 'matplotlib'
+
+    steps:
+      - name: Install Python
+        uses: actions/setup-python@v3
+        with:
+          python-version: '3.x'
+
+      # c.f. https://github.com/actions/download-artifact/issues/3#issuecomment-1017141067
+      - name: Download wheel artifacts from last build on 'main'
+        shell: bash
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          PROJECT_REPO="matplotlib/matplotlib"
+          BRANCH="main"
+          WORKFLOW_NAME="cibuildwheel.yml"
+          ARTIFACT_NAME="wheels"
+
+          gh run --repo "${PROJECT_REPO}" \
+             list --branch "${BRANCH}" \
+                  --workflow "${WORKFLOW_NAME}" \
+                  --json event,status,databaseId > runs.json
+          # Filter on 'push' events to main (merged PRs) that have completed
+          jq --compact-output \
+            '[ .[] | select(.event == "push") | select(.status == "completed") ]' \
+            runs.json > pushes.json
+          # Get id of latest build of wheels
+          RUN_ID=$(
+            jq --compact-output \
+              'sort_by(.databaseId) | reverse | .[0].databaseId' pushes.json
+          )
+          gh run --repo "${PROJECT_REPO}" \
+             download "${RUN_ID}" --name "${ARTIFACT_NAME}"
+
+          mkdir dist
+          mv *.whl dist/
+          ls -l dist/
+
+      - name: Install anaconda-client
+        run: |
+          python -m pip install --upgrade pip setuptools wheel
+          # c.f. https://github.com/Anaconda-Platform/anaconda-client/issues/540
+          python -m pip install git+https://github.com/Anaconda-Server/anaconda-client
+          python -m pip list
+
+      - name: Upload wheels to Anaconda Cloud as nightlies
+        run: |
+          anaconda --token ${{ secrets.ANACONDA_ORG_UPLOAD_TOKEN }} upload \
+            --user scipy-wheels-nightly \
+            dist/matplotlib-*.whl