debian: fix version detection for python3?-nautilus dependency

On buster and stretch use python-nautilus, but /etc/debian_version
contains numeric version, not a codename.

Reported by @0spinboson

debian/rules

@@ -40,7 +40,7 @@ override_dh_install:
 	dh_install --fail-missing
 
 override_dh_gencontrol:
-ifneq ($(shell grep -o 'stretch\|buster' /etc/debian_version),)
+ifneq ($(shell grep -o '^9\|^10' /etc/debian_version),)
 	echo "pythonver:Depends=python" >> debian/qubes-core-agent-nautilus.substvars
 else
 	echo "pythonver:Depends=python3" >> debian/qubes-core-agent-nautilus.substvars