chore([debian]): Update Debian packaging with support for Python 3

Support for Python 3.8 is disabled as it is currently broken in Debian unstable.

Also take this opportunity to run the tests at build time.

--HG--
branch : 0.1

debian/pybuild.testfiles

+test
+tox.ini
+cubicweb_file.egg-info
\ No newline at end of file

debian/rules

@@ -3,5 +3,8 @@
 export PYBUILD_NAME = cubicweb-jsonschema
 export PYBUILD_DISABLE=test
 
+# Python 3.8 is broken in Debian unstable as of 2019-11-21
+export PYBUILD_DISABLE_python3.8 = 1
+
 %:
 	dh $@ --with python2,python3 --buildsystem=pybuild