WSGITask: Patch should be enabled only if waitress egg is available

product/ERP5Type/patches/WSGITask.py

 # -*- coding: utf-8 -*-
 
 import ZPublisher.HTTPRequest
-from waitress.task import WSGITask
+try:
+  from waitress.task import WSGITask
+except ImportError:
+  WSGITask = None
 
-WSGITask_get_environment = WSGITask.get_environment
+# Patch should be enabled only if waitress egg is available
+if WSGITask is not None:
+  WSGITask_get_environment = WSGITask.get_environment
 
-def get_environment(self):
+  def get_environment(self):
     if ZPublisher.HTTPRequest.trusted_proxies == ('0.0.0.0',): # Magic value to enable this functionality
       # Frontend-facing proxy is responsible for sanitising
       # X_FORWARDED_FOR, and only trusted accesses should bypass
@@ -21,4 +26,4 @@ def get_environment(self):
 
     return environ
 
-WSGITask.get_environment = get_environment
+  WSGITask.get_environment = get_environment