Isn't that pretty much the whole point of BSD distributions? And IIRC OpenBSD doesn't even allow binary drivers, everything distributed as part of OpenBSD which runs on the host CPU is open-source (OpenBSD does bundle firmwares blobs)
(otoh it is nice to get more OS support in psutil, it's a neat piece of kit though with an annoying history of API changes)
I was puzzled by the line above regarding source. Like most (all?) BSDs the system is easily checked out and built:
# cd /usr
# cvs -qd firstname.lastname@example.org:/cvs get -rOPENBSD_5_8 -P src
# cd /sys/arch/amd64/conf
# config GENERIC.MP
# cd ../compile/GENERIC.MP
# make depend bsd install
# cd /usr/src
# make obj build
OpenBSD's source tree layout is almost the same as FreeBSD's.
In both trees the source for the various CLI tools lives in src/bin/, src/sbin/, src/usr.bin/, src/usr.sbin/, and a similar structure in src/gnu/ (eg. src/gnu/usr.bin/)
Nothing directly to do with psutil, but related to cross-platform handling of differences between Linux, and various * BSDs (also, a reminder that while vendoring dependencies a la "best practice" go, it's good to keep an eye on upstream - or risk missing fixes).