Make fsevents2 the default implementation on macOS

[BoboTiG]

The primary idea is to get ride of the C extension on macOS.

There is some work on fsevents2 that is is fsevents using PyObjC. Current tests are disabled, and will be fixed with #546.

At term the goal is to remove fsevents.py and watchdog_fsevents.c; and rename fsevents2.py to fsevents.py. A fsevents2.py will be kept for compatibility and display a deprecation warning.
This will allow us to publish one wheel for every platform (#428) and fix a lot of compilation errors happening from time to time.

This issue is here to track the remaining tasks that we are not yet aware.

[samschott]

Uh, this would mean depending on PyObjC which is quite a heavy dependency. It's effectively outsourcing our single extension module to a dependency which covers the entire macOS API.

If you really want to get rid of watchdog_fsevents.c, could you be convinced to use rubicon-objc for the bridging instead? It's based on ctypes, it's light-weight and despite still being at v0.4.0 it is very stable. Though with the recent work which we have put into the extension module, I would not mind keeping it around either.

[BoboTiG]

Of course, given what was done lastly, there is no need to change anything. The C code is quite robust and we have more contributors knowing its internals. And we have wheels for all platforms. So my initial concerns are no longer valid.

What could be interesting, for the future, may be to port fsevents2 to rubicon-objc. And at some point migrating from fsevents to fsevents2. But this is really not a priority and I am fine with the current state of fsevents. Of course, being able to skip C compilation is always a good point IMO.

[samschott]

Thanks for the update, this sounds good to me. I just stumbled upon this issue when considering to port the changes from #815 to fsevents2.