Merge remote-tracking branch 'origin/master' into no-more-load

setup.py

@@ -154,10 +154,10 @@ setup(name="ZODB",
         'test': tests_require,
       },
       install_requires = [
-        'persistent >= 4.1.0',
+        'persistent >= 4.2.0',
         'BTrees >= 4.2.0',
         'ZConfig',
-        'transaction >= 1.4.4',
+        'transaction >= 1.5.0',
         'six',
         'zc.lockfile',
         'zope.interface',

src/ZODB/FileStorage/FileStorage.py

@@ -25,7 +25,6 @@ from struct import pack
 from struct import unpack
 
 from persistent.TimeStamp import TimeStamp
-from six import PY3
 from six import string_types as STRING_TYPES
 from zc.lockfile import LockFile
 from zope.interface import alsoProvides
@@ -2103,13 +2102,10 @@ class FilePool:
 
         This is required if they contain data of rolled back transactions.
         """
+        # Unfortunately, Python 3.x has no API to flush read buffers, and
+        # the API is ineffective in Python 2 on Mac OS X.
         with self.write_lock():
-            if PY3:
-                # Unfortunately, Python 3.x has no API to flush read buffers.
             self.empty()
-            else:
-                for f in self._files:
-                    f.flush()
 
     def close(self):
         with self._cond: