Fix tests with, and depend on, persistent 4.4.

Fixes #213.

CHANGES.rst

@@ -2,13 +2,14 @@
  Change History
 ================
 
-5.4.1 (unreleased)
+5.5.0 (unreleased)
 ==================
 
 - Bump the dependency on zodbpickle to at least 1.0.1. This is
   required to avoid a memory leak on Python 2.7. See `issue 203
   <https://github.com/zopefoundation/ZODB/issues/203>`_.
 
+- Bump the dependency on persistent to at least 4.4.0.
 
 5.4.0 (2018-03-26)
 ==================

setup.py

@@ -14,7 +14,7 @@
 import os
 from setuptools import setup, find_packages
 
-version = '5.4.1.dev0'
+version = '5.5.0.dev0'
 
 classifiers = """\
 Intended Audience :: Developers
@@ -125,7 +125,7 @@ setup(
         'test': tests_require,
     },
     install_requires=[
-        'persistent >= 4.2.0',
+        'persistent >= 4.4.0',
         'BTrees >= 4.2.0',
         'ZConfig',
         'transaction >= 2.0.3',

src/ZODB/tests/util.py

@@ -37,6 +37,11 @@ checker = renormalizing.RENormalizing([
      r"\1"),
     (re.compile('b(".*?")'),
      r"\1"),
+    # Persistent 4.4 changes the repr of persistent subclasses,
+    # and it is slightly different with the C extension and
+    # pure-Python module
+    (re.compile('ZODB.tests.testcrossdatabasereferences.'),
+     ''),
     # Python 3 adds module name to exceptions.
     (re.compile("ZODB.interfaces.BlobError"),
      r"BlobError"),