make systemd support an optional extra

only install sdnotify and systemd-python when the "systemd" extra is requested

maestral/sync/main.py

@@ -42,12 +42,17 @@ IS_NOTIFY = os.getenv("NOTIFY_SOCKET", "")  # set by systemd
 IS_SYSTEMD = os.getenv("INVOCATION_ID", "")  # set by systemd
 
 if IS_SYSTEMD:
-    from systemd import journal
+    try:
+        from systemd import journal
+    except ImportError:
+        IS_SYSTEMD = False
 
 if IS_NOTIFY:
-    import sdnotify
-    system_notifier = sdnotify.SystemdNotifier()
-    print("running under systemd as type=notify")
+    try:
+        import sdnotify
+        system_notifier = sdnotify.SystemdNotifier()
+    except ImportError:
+        IS_NOTIFY = False
 
 # ========================================================================================
 # Logging setup

requirements.txt

@@ -7,5 +7,5 @@ u-msgpack-python
 keyring>=19.0.0
 keyrings.alt>=3.0.0
 Pyro4
-systemd-python; platform_system == 'Linux'
-sdnotify; platform_system == 'Linux'
+systemd-python
+sdnotify

setup.py

@@ -56,9 +56,10 @@ setup(
         "requests",
         "u-msgpack-python",
         "watchdog",
-        "systemd-python; platform_system == 'Linux'",
-        "sdnotify; platform_system == 'Linux'",
     ],
+    extras_require={
+        "systemd": ["systemd-python", "sdnotify"],
+    },
     zip_safe=False,
     entry_points={
       "console_scripts": ["maestral=maestral.cli:main"],