python 3 support: fix errors related to byte/string conversion

[ec1oud]

There were a lot of errors like "a bytes-like object is required,
not 'str'" and the opposite error.

https://stackoverflow.com/questions/33054527/python-3-5-typeerror-a-bytes-like-object-is-required-not-str-when-writing-t
points out that if you open a file in binary mode, you need to write
bytes, not text. It turns out that Popen with universal_newlines
will open the pipe in text mode rather than binary mode; whereas the
regular file opening calls here are already in text mode, so converting
to utf-8 is not required when writing strings (I hope that means
Python already does that by default).

[ec1oud]

I'm not normally much of a python user (so far) so this is guesswork for me, but it seems to work now. I have version 3.7.1 FWIW.

[peterjc]

This was all written and tested under Python 2, and has not been tested under Python 3.

[peterjc]

Cross reference #24

[milahu]

also replace title.encode("utf-8") with title etc,
otherwise strings are formatted as binary strings (b'...') in the output files.

[peterjc]

Thank you for looking at this - I did something similar for what is now v1.1.0:

https://github.com/peterjc/mediawiki_to_git_md/tree/v1.1.0