pyzfs: python3 support (unit tests)

• Updated unit tests to be compatbile with python 2 or 3. In most cases all that was required was to add the 'b' prefix to existing strings to convert them to type bytes for python 3 compatibility.

• There were several places where the python version need to be checked to remain compatible with pythong 2 and 3. Some one more seasoned with Python may be able to find a way to rewrite these statements in a compatible fashion.

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