Ignoring xlib/tx folder, depending on Twisted version.

conftest.py

@@ -1,6 +1,7 @@
 import glob
 import six
 import pytest
+from twisted import version as twisted_version
 
 
 def _py_files(folder):
@@ -21,6 +22,9 @@ def _py_files(folder):
     "scrapy/spider.py",
 ] + _py_files("scrapy/contrib") + _py_files("scrapy/contrib_exp")
 
+if (twisted_version.major, twisted_version.minor, twisted_version.micro) >= (15, 5, 0):
+    collect_ignore += _py_files("scrapy/xlib/tx")
+
 
 if six.PY3:
     for line in open('tests/py3-ignores.txt'):