Update gen-punycode-examples.py for python 3.
gen-punycode-examples.py parses the Sample strings from section 7.1
of rfc3492.txt and generates the punycode_examples.[ch] sources containing
the punycode_examples[].
Python 3 requires that print output be surrounded by parentheses
and the split and join operations have been moved from the "string"
class to built-ins.
This change adds the missing parentheses and switches to the built-in
split and join str operations.
The "string" class is no longer required as an import.
Change-Id: Ic5f341080d2ff2feef692c89e0b28dcbf4e48cb4
