Removed Python 2 and Python 3.5- support

Deprecation of Python versions below 3.6 gives opportunity to unify the
build and install requirements for OpenZFS packages. The minimal
supported Python version is 3.6 as this is the most recent Python
package CentOS/RHEL 7 users can get.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rich Ercolani <rincebrain@gmail.com>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: szubersk <szuberskidamian@gmail.com>
Closes #12925

cmd/dbufstat/dbufstat.in

@@ -27,7 +27,7 @@
 # Copyright (C) 2013 Lawrence Livermore National Security, LLC.
 # Produced at Lawrence Livermore National Laboratory (cf, DISCLAIMER).
 #
-# This script must remain compatible with Python 2.6+ and Python 3.4+.
+# This script must remain compatible with and Python 3.6+.
 #
 
 import sys