Remove defcon requirements

[behdad]

Either ufoLib2 or defcon is required but not both.

[anthrotype]

No, that file is only used to create the test environment. We run out test suite against both ufoLib2 and defcon to make sure we support both. The actual runtime requirements are listed in setup.py install _requires, only those are used automatically when you do pip install ufo2ft.

[anthrotype]

Of course you need one or the other, but since we don't actually import any of the two directly within ufo2ft itself, it's the responsibility of our client to decide which kind of Font object to pass down to our compile methods

[anthrotype]

I hope that makes sense. Feel free to reopen