Update python dependencies

Change-Id: Ic1ba3152160e9255d14ae66881e452a99930b0d8
diff --git a/ndncert/debian/control b/ndncert/debian/control
index c3fdb89..0485ef5 100644
--- a/ndncert/debian/control
+++ b/ndncert/debian/control
@@ -2,7 +2,7 @@
 Priority: extra
 Maintainer: Zhiyi Zhang <zhiyi@cs.ucla.edu>
 Build-Depends: debhelper (>= 8.0.0),
- python (>= 2.7.0),
+ python3 (>= 3.6.0),
  pkg-config (>= 0.26),
  libndn-cxx-dev (>= 0.6.5)
 Standards-Version: 3.9.2