arc_summary.py crashes with -d option when called with Python 3

[scotws]

System information

Type	Version/Name
Distribution Name	Ubuntu
Distribution Version	16.04 LTS
Linux Kernel	4.10.0-38-generic
Architecture	amd64
ZFS Version	0.6.5.9-2
SPL Version	0.6.5.9-1

Describe the problem you're observing

arc_summary.py crashes when called with the option -d with Python 3. The problem does no occur with Python 2.

Describe how to reproduce the problem

Run python3 arc_summary.py -d

Include any warning/errors/backtraces from the system logs

Traceback (most recent call last):
  File "/usr/sbin/arc_summary.py", line 1148, in <module>
    main()
  File "/usr/sbin/arc_summary.py", line 1144, in main
    page(Kstat)
  File "/usr/sbin/arc_summary.py", line 1035, in _tunable_summary
    description_list = p.communicate()[0].strip().split('\0')
TypeError: a bytes-like object is required, not 'str'

Tentative Diagnosis

Python 3 changed the way encoding is handled with files (see https://docs.python.org/3.0/whatsnew/3.0.html#text-vs-data-instead-of-unicode-vs-8-bit). It is probable that the pipe called here now returns the wrong sort of data.

[scotws]

After a bit of testing: Python2 returns a string as the first element of the tuple produced by p.communicate() here, but Python3 returns bytes. Will take a look at fixes and figure out a PR to fix this.