Include external projects as subtrees #854

Job	Run time
Test python stubs	44s
Build (ubuntu-latest)	54s
Build (macos-latest)	59s
Build (windows-latest)	3m 56s
linux (x86_64)	1m 11s
sdist	24s
macos (x86_64)	52s
Ensure Python API crate version is in sync	5s
linux (x86)	1m 32s
macos (aarch64)	56s
linux (aarch64)	1m 34s
windows (x64)	2m 27s
linux (armv7)	1m 47s
windows (x86)	2m 21s
linux (s390x)	1m 33s
linux (ppc64le)	1m 33s
Release	0s
	22m 48s

Provide feedback