remove scary warning when pillow is missing

xpra/client/mixins/window_manager.py

@@ -31,7 +31,7 @@
     )
 from xpra.util import (
     envint, envbool, typedict,
-    make_instance, updict, repr_ellipsized, u, noerr,
+    make_instance, updict, repr_ellipsized, u, noerr, first_time,
     )
 from xpra.client.mixins.stub_client_mixin import StubClientMixin
 from xpra.log import Logger
@@ -683,8 +683,15 @@ def set_tray_icon(self):
     def _window_icon_image(self, wid, width, height, coding, data):
         #convert the data into a pillow image,
         #adding the icon overlay (if enabled)
-        from PIL import Image  # @UnresolvedImport
         coding = bytestostr(coding)
+        try:
+            # pylint: disable=import-outside-toplevel
+            from PIL import Image
+            return Image
+        except ImportError:
+            if first_time("window-icons-require-pillow"):
+                log.info("showing window icons requires python-pillow")
+            return None
         iconlog("%s.update_icon(%s, %s, %s, %s bytes) ICON_SHRINKAGE=%s, ICON_OVERLAY=%s",
                 self, width, height, coding, len(data), ICON_SHRINKAGE, ICON_OVERLAY)
         if coding=="default":