prevent 'import scrapy' from failing when twisted module is not available, also moved twisted 2.5.0 monkeypatch into a more specific module name

scrapy/__init__.py

@@ -13,8 +13,7 @@ if sys.version_info < (2,5):
     sys.exit(1)
 
 # monkey patches to fix external library issues
-from scrapy.xlib.patches import apply_patches
-apply_patches()
+from scrapy.xlib import twisted_250_monkeypatches
 
 # optional_features is a set containing Scrapy optional features
 optional_features = set()

scrapy/xlib/patches.py

@@ -1,13 +1,9 @@
 """
 Monkey patches for supporting Twisted 2.5.0
+
+NOTE: This module must not fail if twisted module is not available.
 """
 
-import twisted
-
-def apply_patches():
-    if twisted.__version__ < '8.0.0':
-        add_missing_blockingCallFromThread()
-
 # This function comes bundled with Twisted 8.x and above
 def add_missing_blockingCallFromThread():
     import Queue
@@ -42,3 +38,11 @@ def add_missing_blockingCallFromThread():
 
     from twisted.internet import threads
     threads.blockingCallFromThread = blockingCallFromThread
+
+try:
+    import twisted
+    if twisted.__version__ < '8.0.0':
+        add_missing_blockingCallFromThread()
+except ImportError:
+    pass
+