"make clean" fails in my tree.  There's an inscrutable new directory,
"@test", which is chmod 400, and therefore "find" isn't allowed to
descend into it, so it fails and aborts the clean early.  I don't know
what the deal is here, but I patched the rules for "clean" and
"pycremoval" so they chmod "@test" temporarily to 755 before running and
back to "400" afterwards.

A better fix: get rid of this "chmod 400 @test" nonsense.  But I'm too
lazy to investigate why it was done in the first place.  Or tell me what
I'm doing wrong in the first place...