ci: Separate release-please PRs, and set a better title (#520)

Small change so that `release-please`'s PR include a `(py)` tag in their
title.

I also enabled separated PRs for the packages, since we don't want to
release `tket2-eccs` unless needed.

release-please-config.json

@@ -4,6 +4,8 @@
     "bump-minor-pre-major": true,
     "bump-patch-for-minor-pre-major": true,
     "initial-version": "0.0.0",
+    "separate-pull-requests": true,
+    "pull-request-title-pattern": "chore(py): release${component} ${version}",
     "packages": {
         "tket2-py": {
             "release-type": "python",