Python-rtmidi

Project infrastructure:

- Declared/documented Python 3.9 support and updated CI.
- Dropped official support for Python 3.5.
- Updated Python versions for building macOS binary wheels.

Fixes:

- Revert to old way of reading version number via `exec` (see
8d9a8f9 for background info).

Fixes:

- Fixed `setup.py` to work with Python 2 again, though it is not
officially supported anymore (70, 72).

Fixes:

- Fixed error when trying to import version number from `rtmidi.release`
when Python is running with `-OO` optimization enabled (69).

Project infrastructure:

- Moved distribution meta data to `setup.cfg`, added
`rtmidi/version.py` and removed `rtmidi/release.py` (69).

Enhancements:

- Added more helpful aliases for MIDI events/controllers constants.

Examples:

- Added `ccstore` advanced example to show how to remember last seen
controller change values (64).

Changes:

- Suppress RtMidi warnings to stderr, so that warnings issued in
the constructor of the `RtMidiIn/Out` C++ class instances before
the default error handler function can be attached, don't end
up in the output.

The suppression of RtMidi warnings can be disabled at compile time by
setting the pre-compiler define `__RTMIDI_SUPPRESS_WARNINGS__` via a
command line option to `setup.py`.

As before, RtMidi warnings are turned into Python a `UserWarning` as soon
as the default error handler is attached, but this can only happen after
the RtMidi C++ class has been instantiated (59).

- Allow deletion of internal C++ RtMidiIn/Out instance via new `delete`
method of `rtmidi.MidiIn` and `rtmidi.MidiOut` instances (but see warning
in docstring!). Also added a `is_deleted` property to both classes (60).