Me too. This seems to be an instance of upstream bug #3225 ( https://github.com/kivy/kivy/issues/3225 ). There is apparently something which needs to be cleaned up between the python2 and python3 builds that isn't. The resolution of #3225 points to a debina/rules file which does some additional cleanup between builds ( https://github.com/thopiekar/kivy_fork/blob/03abc51fad6ccf5b11bb37d678ae334540262683/rules ).
I can confirm that building from the debian sources with the linked rules file via: # Debian stretch sources and system apt-get source kivy cd kivy-1.9.1 vim debian/rules dpkg-buildpackage -rfakeroot -uc -us -tc -i produces a functional python3-kivy package.
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