Darkdetect

A new release of darkdetect is ready. The most prominent new feature is a listener for macOS, finally, thanks to the efforts of zwimer. In addition, from this release the support for Python 2.7 and any versions older than 3.6 is dropped. Users of older Python versions can continue using `darkdetect<=0.7.*`.

Changelog:
- macOS: add `listener` function (issue 25, PR 30 by zwimer)
- using the macOS listener requires an extra that can be installed via `pip install darkdetect[macos-listener]`
- Linux: add initial support for freedesktop standard on gnome (PR 22 by raghavdhingra24)
- Port build chain from `setup.py` to `pyproject.toml` (PR 28 by zwimer)
- Drop support for Python <3.6

It's time for a patch release. The main change is that compatibility with Python 2.7 should now be restored.

Changelog:
- restore compatibility with Python 2.7 (inadvertently removed in 20).
- add `__main__` module to make the package callable from a terminal as `python -m darkdetect`.

This release contains an important change for Linux: from now on, all the themes that have `-dark` (case-insensitive) in their names will be detected as `'Dark'` by `darkdetect`. Thanks to DoctorDalek1963 for this contribution!

Changelog:
- Linux: generalise detection to any theme with `'-dark'` in `theme.lower()` (initially suggested in 17 by HetDaftary, PR 24 by DoctorDalek1963. Thanks to larsoner for his support).
- macOS: do not use the deprecated `distutils.version` to check OS version compatibility. The needed logic was reimplemented (issue 18 by Carreau).

Time for a new release with an exciting new feature: listeners for theme changes for Windows and Linux! Many thanks to TransparentLC for having implemented those.

Changelog:
- New `callback` API to dynamically detect when the theme is changed on Windows and Linux (issue 14, PR 20 by TransparentLC)
- Windows: avoid triggering `ValueError` when the package is imported on Windows Server (PR 21 by PierreRaybaut)
- Windows: switch from `sys` to `platform` to detect the OS version

A few minor bug fixes:
- Windows: the package could have raised `FileNotFoundError` in some headless systems.
- In those cases, we catch the exception and return `None` as if the dark mode were not available
- macOS: fix use of the package in hardened programs (see PR 15 for the bug report)
- Windows: precautionary changes to support Windows 11 (PR 16 by hussain5416)

- Add compatibility with macOS Big Sur on arm64 (Apple Silicon) (issue 12 raised and debugged by psobolewskiPhD)
- Restored compatibility with Python 2.7 on macOS Big Sur