debian: drop python2 in build deps

QubesOS/qubes-issues#5297

debian/control

@@ -6,7 +6,7 @@ Build-Depends:
     libpam0g-dev,
     libqubes-rpc-filecopy-dev (>= 3.1.3),
     libvchan-xen-dev,
-    python,
+    python3,
     python3-setuptools,
     debhelper,
     quilt,