Due to checking in error outputs these tests only work reliably on stable and
not on intermediate version between our MSRV (currently 1.48) and the current
stable version.
Hence this simplifies things to run only MSRV-compatible tests for the MSRV
builds, anything else for stable builds except for those tests which require the
nightly feature, i.e. the `Ungil` being distinct from the `Send` trait.
Finally, `not_send3` is disabled when using the nightly feature since while `Rc`
is not send, it also not GIL-bound and hence can be passed into `allow_threads`
as it does not actually spawn a new thread.
