CI: activate github actions on 1.1.x (PR only) - DO NOT MERGE
IIUC without changes to other scripts, activating GitHub actions for merge would push the docs. so this is only for PRs

If works. will open PR against master instead. and backport.

xref https://github.com/pandas-dev/pandas/pull/34800#issuecomment-644669496
creating a PR using same branch re-triggered CI here. Github actions (and Travis) were green