Screen-brightness-control

* Fix 13 where some laptop displays were being returned by `windows.VCP.iter_physical_monitors`
* `windows.VCP.iter_physical_monitors` will now only return handles for monitors with the `win32con.DISPLAY_DEVICE_ATTACHED_TO_DESKTOP` StateFlag (see [this comment](https://github.com/Crozzers/screen_brightness_control/issues/13#issuecomment-956567787) for explanation)

* Merge PR 12

* Fix `linux.DDCUtil.set_brightness` expiring non-existent cache keys
* Refactor `linux.XRandr.get_display_info` and `linux.DDCUtil.get_display_info` to be more readable
* All calls to `ddcutil` executable now have stderr suppressed
* Minor optimizations and tweaks to reduce pylint warnings

* Added `linux.SysFiles` as a way of addressing laptop displays without requiring a 3rd party program
* Added tests for `windows` and `linux` sub-modules
* Remove remaining deprecated functions from 10 (`windows.VCP.get_monitor_caps`, `linux.get_brightness_from_sysfiles`)

* Fix TypeError in `windows.VCP.set_brightness` when calling without specifying the `display` kwarg

Added/Changed:
* `set_brightness` now returns `None` by default. To enable old behaviour pass the kwarg `no_return=False` when calling it

Removed:
* `windows.set_brightness`
* `windows.get_brightness`
* `windows.list_monitors`
* `windows.WMI.get_display_names`
* `windows.VCP.get_display_names`
* `linux.set_brightness`
* `linux.get_brightness`
* `linux.list_monitors`
* `linux.Light.get_display_names`
* `linux.XRandr.get_display_names`
* `linux.DDCUtil.get_display_names`

Fixed:
* Fix `fade_brightness` not working on Linux systems with only `xbacklight` installed

Deprecated:
* `windows.VCP.get_monitor_caps`
* `linux.get_brightness_from_sysfiles`