fix autopkgtest

debian/tests/control

-Test-Command: cp -r tests $AUTOPKGTEST_TMP && cd $AUTOPKGTEST_TMP/tests && ./manage.py test -v2 testapp
+Test-Command: cp -r tests $AUTOPKGTEST_TMP && cd $AUTOPKGTEST_TMP/tests && python3 manage.py test -v2 testapp
 Restrictions: allow-stderr
 Features: test-name=testsuite