Switch to Python3

Switch to Python3

Support for Python2.7 in Linux distributions is being phased out. As an
example, some Linux distributions do not provide Python2.7 versions of
some of the Python packages fio Python scripts relies on. Hence switch
to Python3. Most of the Python code changes in this patch have been
generated by running the Python scripts through 2to3, the Python2 to
Python3 converter from python.org.

Notes:
- Python is only used by fio test scripts and not by fio itself.
- Older versions of RHEL / CentOS 6 do not support Python3.
  Python3 binaries for RHEL 6 / CentOS 6 are available e.g. in the EPEL
  repository (https://fedoraproject.org/wiki/EPEL).

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>