debian: really drop python2 build

Fixes d302aea "Drop Python2 support"
QubesOS/qubes-issues#5297

debian/rules

@@ -20,7 +20,7 @@ include /usr/share/dpkg/default.mk
 
 # main packaging script based on dh7 syntax
 %:
-	dh $@ --with python2,python3,sphinxdoc --buildsystem=pybuild
+	dh $@ --with python3,sphinxdoc --buildsystem=pybuild
 
 override_dh_auto_build: export http_proxy=127.0.0.1:9
 override_dh_auto_build: export https_proxy=127.0.0.1:9