Mark linux.Light as deprecated.

Light has been unmaintained for a while (since Apr 2023 at least) and the repo was recently deleted.
There have been attempts from package maintainers to fork the project and maintain a build, but there's no indication yet
of a new maintainer.

I've marked this as deprecated but without a planned removal date/version since the dust is still settling. I'll add
a removal date if it becomes clear that no maintainer will step forwards.

New code should use `linux.SysFiles` instead

docs/source/extras/Installing On Linux.py

@@ -51,19 +51,19 @@
 Here is an outline of all of the external programs that `screen_brightness_control` can call upon:
 
 Program       | Works on laptop displays | Works on external monitors | Per-display brightness control        | Requires Special Permissions After Install
---------------|--------------------------|----------------------------|---------------------------------------|-------------------------------------------------------------
-ddcutil       | No                       | Yes (slowest) [1]          | Yes                                   | Read/write access for `/dev/i2c*` [2]
-xrandr        | Yes                      | Yes           [3]          | Yes                                   | No
-light         | Yes                      | No                         | Yes                                   | User must be in the `video` group [3]
-[No program]  | Yes                      | Yes (slow)                 | Yes                                   | Read/write access for `/dev/i2c*` and `/sys/class/backlight`
+--------------|--------------------------|----------------------------|---------------------------------------|-------------------------------------------------------------------
+ddcutil       | No                       | Yes (slowest) [1]          | Yes                                   | Read/write access for `/dev/i2c*` (see [above](#desktop-displays))
+xrandr [2]    | Yes                      | Yes                        | Yes                                   | No
+light [3]     | Yes                      | No                         | Yes                                   | User must be in the `video` group [4]
+[No program]  | Yes                      | Yes (slow)                 | Yes                                   | See [above](#without-using-a-3rd-party-program)
 
 #### Footnotes
 [1] While both DDCUtil and the 1st party `linux.I2C` class do similar things over the same interface (I2C),
 DDCUtil also supports communicating with monitors that implement the [Monitor Control Command Set over USB](https://www.ddcutil.com/usb)
 
-[2] Read/write access for the `i2c` bus can be granted via the steps outlined in the [Desktop displays](#desktop-displays) section or by running the script using `sudo`.
+[2] Xrandr has two key limitations. It doesn't support Wayland and it doesn't actually change the backlight of the display, it just changes the brightness by applying a filter to the pixels to make them look dimmer/brighter.
 
-[3] Xrandr does not actually change the backlight of the display, it just changes the brightness by applying a filter to the pixels to make them look dimmer/brighter.
+[3] This method is deprecated as the original Light project was archived in April 2023 and the repository has since been deleted. Unofficial packages are still available for some distros.
 
 [4] You can add yourself to the video user group by running `sudo usermod -a -G video [your username]` and then logging out and back in again for the changes to take effect.
 

screen_brightness_control/linux.py

@@ -5,6 +5,7 @@
 import operator
 import os
 import re
+import warnings
 import time
 from typing import List, Optional, Tuple
 
@@ -439,10 +440,9 @@ class Light(BrightnessMethod):
     3rd party tool that can control brightness levels for e-DP displays.
 
     .. warning::
-       As of April 2nd 2023, the official repository for the light project has
-       [been archived](https://github.com/haikarainen/light/issues/147) and
-       will no longer receive any updates unless another maintainer picks it
-       up.
+       The official repository was archived in April 2023 and has since been deleted.
+       Due to [having no maintainer](https://github.com/haikarainen/light/issues/147)
+       or official packages, this feature has been marked as deprecated.
     '''
 
     executable: str = 'light'
@@ -456,6 +456,14 @@ def get_display_info(cls, display: Optional[DisplayIdentifier] = None) -> List[d
         Works by taking the output of `SysFiles.get_display_info` and
         filtering out any displays that aren't supported by Light
         '''
+        warnings.warn(
+            (
+                'Light is unmaintained and has been deprecated.'
+                ' Please use `SysFiles` instead'
+            ),
+            DeprecationWarning
+        )
+
         light_output = check_output([cls.executable, '-L']).decode()
         displays = []
         index = 0
@@ -479,6 +487,13 @@ def get_display_info(cls, display: Optional[DisplayIdentifier] = None) -> List[d
 
     @classmethod
     def set_brightness(cls, value: IntPercentage, display: Optional[int] = None):
+        warnings.warn(
+            (
+                'Light is unmaintained and has been deprecated.'
+                ' Please use `SysFiles` instead'
+            ),
+            DeprecationWarning
+        )
         info = cls.get_display_info()
         if display is not None:
             info = [info[display]]
@@ -489,6 +504,13 @@ def set_brightness(cls, value: IntPercentage, display: Optional[int] = None):
 
     @classmethod
     def get_brightness(cls, display: Optional[int] = None) -> List[IntPercentage]:
+        warnings.warn(
+            (
+                'Light is unmaintained and has been deprecated.'
+                ' Please use `SysFiles` instead'
+            ),
+            DeprecationWarning
+        )
         info = cls.get_display_info()
         if display is not None:
             info = [info[display]]