install: use omemo in install_requires only for platforms supported upstream

setup.py

@@ -20,6 +20,7 @@
 
 from setuptools import setup, find_packages
 import os
+import platform
 
 NAME = 'sat'
 
@@ -50,9 +51,12 @@ install_requires = [
     'urwid >= 1.2.0',
     'urwid-satext >= 0.6.1',
     'wokkel >= 0.7.1',
-    'omemo',
 ]
 
+if platform.system() in ('Linux', 'Windows'):
+    # python-omemo is currently supported only on those platforms
+    install_requires.append('omemo')
+
 DBUS_DIR = 'dbus-1/services'
 DBUS_FILE = 'misc/org.salutatoi.SAT.service'
 with open(os.path.join(NAME, 'VERSION')) as f: