diversion: treat evdev as an optional dependency

evdev was imported unconditionally on every OS except macOS and Windows,
i.e. the code assumed "not macOS and not Windows" means Linux. On any
other platform the import is a hard failure at startup.

evdev is also genuinely optional: setup.py only installs it when
platform_system == "Linux", and packagers may make it optional. Select on
whether the module imports rather than on the OS name.

click_uinput() dereferenced evdev.ecodes before calling simulate_uinput(),
so it raised AttributeError rather than degrading; click() has no handler,
so that escaped to the caller. Bail out early there and in setup_uinput()
when evdev is absent. simulate_scroll() was already safe, but only because
setup_uinput()'s blanket except swallowed the AttributeError.

lib/logitech_receiver/diversion.py

@@ -21,7 +21,6 @@ import logging
 import math
 import numbers
 import os
-import platform
 import socket
 import struct
 import subprocess
@@ -39,12 +38,13 @@ import yaml
 
 from keysyms import keysymdef
 
-# There is no evdev on macOS or Windows. Diversion will not work without
-# it but other Solaar functionality is available.
-if platform.system() in ("Darwin", "Windows"):
-    evdev = None
-else:
+# evdev is not available on every platform, and is an optional dependency
+# elsewhere. Diversion will not work without it but other Solaar
+# functionality is available.
+try:
     import evdev
+except ImportError:
+    evdev = None
 
 from .common import NamedInt
 from .hidpp20 import SupportedFeature
@@ -257,6 +257,8 @@ else:
 
 def setup_uinput():
     global udevice
+    if evdev is None:
+        return False
     if udevice is not None:
         return udevice
     try:
@@ -337,6 +339,8 @@ def simulate_key(code, event):  # X11 keycode but Solaar event code
 
 
 def click_uinput(button, count):
+    if evdev is None:
+        return False
     if isinstance(count, int):
         for _ in range(count):
             if not simulate_uinput(evdev.ecodes.EV_KEY, button[1], 1):