Don't die horribly if xephyr fails to start

As seen in https://travis-ci.org/qtile/qtile/jobs/39096012, if xephyr fails to
start, we generate a long chain of exceptions, because we're using testwindows
before it is initalized. We should initialize it first.
Use to_atoms instead of unpacking struct
Change .get_property(unpack=...) to be int, use to_atoms/string

tych0: minor correction for the bug we were seeing in
1739627aa6534b0a88a45cd98587a52753e857e8.

Closes #536
try pypy3 tests
try build with source xcffib