Require transaction >= 1.4.1 on Python 3

Because our test suite triggers that 'RuntimeError: dictionary changed
size during iteration' error sporadically.

buildout.cfg

@@ -12,7 +12,7 @@ zope.interface = 4.0.3
 ZConfig = 3.0.1
 BTrees = 4.0.5
 persistent = 4.0.6
-transaction = 1.4.0
+transaction = 1.4.1
 zdaemon = 4.0.0a1
 
 [test]

setup.py

@@ -30,6 +30,8 @@ if sys.version_info < (2, 6):
     print("This version of ZODB requires Python 2.6 or higher")
     sys.exit(0)
 
+PY3 = sys.version_info >= (3,)
+
 # The (non-obvious!) choices for the Trove Development Status line:
 # Development Status :: 5 - Production/Stable
 # Development Status :: 4 - Beta
@@ -124,7 +126,7 @@ setup(name="ZODB",
         'BTrees',
         'ZConfig',
         'persistent',
-        'transaction',
+        'transaction >= 1.4.1' if PY3 else 'transaction',
         'six',
         'zc.lockfile',
         'zdaemon >= 4.0.0a1',