ci: add release-please (#22)

Trying it out on a Python project ... hopefully it's nice and simple? I
made a fine-grained token for all **stactools-packages** in case we want
to re-use this pattern.

Author: gadomski

.github/workflows/release-please.yml

@@ -0,0 +1,20 @@
+on:
+  push:
+    branches:
+      - main
+
+permissions:
+  contents: write
+  issues: write
+  pull-requests: write
+
+name: release-please
+
+jobs:
+  release-please:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: googleapis/release-please-action@v4
+        with:
+          token: ${{ secrets.RELEASE_PLEASE_TOKEN }}
+          release-type: python