I finally cracked a nut that was at the root of our cross-compile woes with
pyo3, which was the root cause of python3-cryptography not building.

For those interested, my WIP branch is:
https://git.openembedded.org/meta-openembedded-contrib/log/?h=timo/python3-cryptography_36.0.1

If someone is interested, figuring out why python3-setuptools-rust-native
is not aware of where the crate:// fetcher puts things would be helpful.
For now the hack is:

CARGO_DISABLE_BITBAKE_VENDORING = "1"

This is purely a hack and should not be necessary.

Read the commit log for info.

This is not the final form of this effort.

--Tim