packaging: use /usr/bin/python3 shebang for scripts in Debian

"hg" script is already correct because it is handled by setup.py but
"hg-ssh" will be rewritten by dh_python into "/usr/bin/python" which is
not wanted as we target Python 3. By passing --shebang=/usr/bin/python3
to dh_python3, we force shebangs to be set with this value.

contrib/packaging/debian/rules

@@ -14,7 +14,7 override_dh_auto_test:
 	http_proxy='' dh_auto_test -- TESTFLAGS="-j$(CPUS)"
 
 override_dh_python3:
-	dh_python3
+	dh_python3 --shebang=/usr/bin/python3
 	find debian/mercurial/usr/share -type d -empty -delete
 
 override_dh_install: