gftools-add-font: support py3

[m4rc1e]

Script is still backwards compatible with py2.7+. I've only tested it on a handful of families.

I've had to implement a work around for the cmp function since py3 removed it. I used the python porting guide as a reference.

@garretrieger are you folks running a verion of python which is greater than or equal to 2.7? if not, I'll remove the use of functools.

I'm hoping to fix #89 soon. I may need help coordinating the fix so I don't break GF's internals.

[davelab6]

Folks are running v2.7.13

[davelab6]

https://twitter.com/FontTools/status/1074788792832266240 says

Important announcement! From January 2019, FontTools joins https://python3statement.org initiative and will begin to gradually drop support for Python 2. Only bug fixes are scheduled for py27 in the next 6 months, new features will require Python 3! 🐍🐍🐍 -

We should probably match that

[davelab6]

@eliheuer please review

[m4rc1e]

@eliheuer I'm done.

I've done the following since the first pr:

• drop google-apputils because it's deprecated. Absl-py has replaced it
• utils.download_file now uses requests instead of urllib (diff interfaces between py2 and py3)
• py2 cmp function moved to utils

[eliheuer]

Nice, looking at this now.

[eliheuer]

It works, but for some reason bin/gftools-add-font.py is missing the python3 header. I'll merge this and make another PR. Thanks Marc!