automating maintenance with Github actions (aws-cloudformation#1792)

IAM actions currently required:
https://github.com/aws-cloudformation/cfn-python-lint/blob/1ad5de337c3a388d949c30b738e48d15fae2092b/scripts/update_specs_services_from_ssm.py#L131
https://github.com/aws-cloudformation/cfn-python-lint/blob/1ad5de337c3a388d949c30b738e48d15fae2092b/scripts/update_specs_from_pricing.py#L90

* reducing frequency to avoid ThrottlingExceptions

.github/workflows/maintenance.yaml

@@ -0,0 +1,28 @@
+on:
+  schedule:
+    - cron: '*/35 * * * *'
+jobs:
+  job:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-python@v2
+        with:
+          python-version: 3
+      - run: |
+          pip install -e .
+          rm -rf src/cfnlint/data/DownloadsMetadata/*
+          cfn-lint --update-iam-policies
+          cfn-lint --update-documentation
+          scripts/update_specs_services_from_ssm.py
+          scripts/update_specs_from_pricing.py
+          cfn-lint --update-specs
+        env:
+          AWS_ACCESS_KEY_ID: ${{ secrets.AWS_ACCESS_KEY_ID }}
+          AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
+      - uses: peter-evans/create-pull-request@v3
+        with:
+          commit-message: |
+            autogenerated maintenance
+          delete-branch: true
+          title: autogenerated maintenance