Python 2 and 3 compatibility

With Python 2 (slowly) approaching EOL and its removal from distribitions
already being planned (Fedora), the existing Python 2 code needs to be
transitioned to Python 3.  This patch stack updates the Python code to
be compatible with Python 2.7, 3.4, 3.5, 3.6, and 3.7.

Reviewed-by: John Ramsden <johnramsden@riseup.net>
Reviewed-by: Neal Gompa <ngompa@datto.com>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Wren Kennedy <john.kennedy@delphix.com>
Reviewed-by: Antonio Russo <antonio.e.russo@gmail.com>
Closes #8096