Require pyobjc on macOS

[olebole]

On macOS, there are three possible ways to create interactive graphics: aqua, x11, matplotlib.
By default, this should probably be aqua, and aqua requires additionally the pyobjc package and its dependencies.

One question however is whether this should be required. This PR makes it required as a base for testing and discussion.

Cc @monodera

This could fix #156

[olebole]

Just checked disk usage on a fresh Mac/M1:

• without pyobjc 49 MB
• with pyobjc 77 MB

[monodera]

A potential issue is that the current PyObjC supports Python 3.7 or later, while PyRAF support includes Python 3.6.

The PyObjC web site (https://pyobjc.readthedocs.io/en/latest/) says

PyObjC 9.2 is the last version supporting Python 3.7. PyObjC 8.5 is the last version supporting Python 3.6. PyObjC 5.3 is the last version supporting Python 2. These versions are no longer supported.

PyObjC is regularly tested on macOS 10.14 and should work on macOS 10.9 or later for the i386 and x86_64 architectures.

Perhaps, pip can resolve to install an appropriate version, but they may not be officially supported anymore.

[olebole]

Python 3.6 isn't officially supported anymore either; EOL was 2021. And the oldest supported version of Numpy is currently 1.23, which requires Python 3.8.
I would keep still Python 3.6 as minversion in setup.cfg for PyRAF; however even now this depends on outdated versions. As long as it works, people can install it.
Alternative would be to have installation instructions that pyobjc is "strongly recommended" on macOS, and/or have an optional "aqua" dependency (and recommend to pip install pyraf[aqua] on a Mac).
However, if there is a working pyobjc available for 3.6 (supported or not), I would probably rather prefer to have it a requirement on Mac, as this removes disambiguities here.