Fix string instead of bytes-like object error.

[arenius]

Running x12norm with python3 I received the following error:

Traceback (most recent call last):
File "/usr/local/bin/x12norm", line 11, in
sys.exit(main())
File "/usr/local/lib/python3.6/dist-packages/pyx12/scripts/x12norm.py", line 71, in main
fd_out.write(seg_data.format() + eol)
TypeError: a bytes-like object is required, not 'str'

fd_out is a "tempfile.TemporaryFile()".

The tempfile docs state:
The mode parameter defaults to 'w+b' so that the file created can be read and
written without being closed. Binary mode is used so that it behaves
consistently on all platforms without regard for the data that is stored.

In other parts of the x12norm code mode 'w' is set explicitly so I think
it is okay to do so for the tempfile as well. This commit does that.

[coveralls]

Coverage remained the same at 66.826% when pulling 537493d on arenius:master into 9285ec1 on azoner:master.

[azoner]

master still targets python2. The branch py3-port has all tests passing on python3. Does this error still occur on that branch?