2826: ci: run checks for all platforms on PR r=adamreichold a=davidhewitt
I've been struggling a little to merge PRs with the new bors workflow; overall I think the lighter PR workflow is better but the number of combinations of older pythons / platforms not covered makes it easy for the bors step to fail.
This PR tries to improve the situation by merging the `clippy` and `check-target` job and running it for all supported platforms on PR. Hopefully if these are green, then there's high likelihood that tests will build and should pass unless there's logic errors.
While creating this I found a build error on PyPy which made me notice we can support `PyList::get_item_unchecked` for PyPy, so I added it.
Co-authored-by: David Hewitt <1939362+davidhewitt@users.noreply.github.com>