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/arcstat/arcstat.in

@@ -47,7 +47,7 @@
 # @hdr is the array of fields that needs to be printed, so we
 # just iterate over this array and print the values using our pretty printer.
 #
-# This script must remain compatible with Python 2.6+ and Python 3.4+.
+# This script must remain compatible with Python 3.6+.
 #
 
 import sys