Make `PyQt5` an optional dependency

Some other projects use this as a dependency for communication with pinterest but don't require GUI functionality from it. Since PyQt5 is a somewhat heavy dependency (and currently has issues installing on some ARM devices like mac M1s) can that be made an optional dependency?

Something like the following in setup.py:

setup(
    ...
    extras_require={
        "gui": ["PyQt5"],
    },

Then anybody wanting to use the GUI can install through pip like:

$ pip install 'py3-pinterest[gui]'

Only noticed because the default branch currently has PyQt5 in its setup.py but doesn't use it anywhere yet.

bstoilov / py3-pinterest

Make `PyQt5` an optional dependency #166