Deprecate XRandr method

XRandr will be removed in the next release.
There are a couple of reasons to deprecate XRandr:

1. X11 only
Xrandr doesn't support Wayland, and X11 is EOL. Gnome, Fedora and Ubuntu have dropped X11, and KDE is going to sometime soon.
70% of KDE users are on Wayland anyway.

2. It doesn't change backlight, only gamma
This has been a longstanding issue which is that xrandr doesn't change the backlight, it applies a gamma filter. This meant you could
accidentally dim your screen "twice" with different brightness methods and not realise how to fix it. This is confusing behaviour
and isn't what this library should do.

3. Functionality has been superseded
XRandr simply isn't needed any more. The SysFiles and I2C classes both cover pretty much all displays, with DDCUtil covering USB displays.
It's just not required anymore.

I'll probably publish a release with the deprecation warning in place and then remove it in the next one, or in a few months, whichever comes first

Author: Crozzers

docs/source/extras/Installing On Linux.py

@@ -60,7 +60,8 @@
 [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] 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.
+[2] The use of Xrandr by this library is deprecated and will be removed in a future release.
+Xrandr 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.
 
 
 ## Install Instructions

screen_brightness_control/linux.py

@@ -7,6 +7,7 @@
 import re
 import time
 from typing import List, Optional, Tuple
+import warnings
 
 from . import filter_monitors, get_methods
 from .exceptions import I2CValidationError, NoValidDisplayError, format_exc
@@ -431,7 +432,15 @@ def set_brightness(cls, value: IntPercentage, display: Optional[int] = None):
 
 
 class XRandr(BrightnessMethodAdv):
-    '''collection of screen brightness related methods using the xrandr executable'''
+    '''
+    .. warning:: Deprecated
+       The XRandr brightness method has been deprecated and will be removed in a future release.
+       XRandr only works on X11, which is starting to be dropped by most distributions.
+       Furthermore, the xrandr executable doesn't change backlight brightness like the other methods here,
+       but changes the gamma instead, making the behaviour inconsistent with the rest of the library.
+
+    Collection of screen brightness related methods using the xrandr executable
+    '''
 
     executable: str = 'xrandr'
     '''the xrandr executable to be called'''
@@ -549,6 +558,7 @@ def get_display_info(cls, display: Optional[DisplayIdentifier] = None, brightnes
 
     @classmethod
     def get_brightness(cls, display: Optional[int] = None) -> List[IntPercentage]:
+        warnings.warn('The xrandr method is deprecated. Please use another brightness method', DeprecationWarning)
         monitors = cls.get_display_info(brightness=True)
         if display is not None:
             monitors = [monitors[display]]
@@ -558,6 +568,7 @@ def get_brightness(cls, display: Optional[int] = None) -> List[IntPercentage]:
 
     @classmethod
     def set_brightness(cls, value: IntPercentage, display: Optional[int] = None):
+        warnings.warn('The xrandr method is deprecated. Please use another brightness method', DeprecationWarning)
         value_as_str = str(float(value) / 100)
         info = cls.get_display_info()
         if display is not None: