windows: fix failures due to low-precision of time.time()

This fixes the following random failure:

Failure in test checkBaseHistory (ZODB.tests.testDemoStorage.DemoStorageTests)
Traceback (most recent call last):
  File "unittest\case.py", line 384, in _executeTestPart
    function()
  File "ZODB\tests\testDemoStorage.py", line 96, in checkBaseHistory
    self._checkHistory(base_only())
  File "ZODB\tests\HistoryStorage.py", line 46, in _checkHistory
    self.assertLess(a, b)
  File "unittest\case.py", line 1026, in assertLess
    self.fail(self._formatMessage(msg, standardMsg))
  File "unittest\case.py", line 508, in fail
    raise self.failureException(msg)
AssertionError: 1465433823.260304 not less than 1465433823.260304

src/ZODB/tests/HistoryStorage.py

@@ -17,7 +17,8 @@
 all these tests.
 """
 
-from time import time
+import sys
+from time import time, sleep
 from ZODB.tests.MinPO import MinPO
 
 class HistoryStorage:
@@ -31,6 +32,9 @@ def _checkHistory(self, data):
         self.assertRaises(KeyError,self._storage.history,oid)
         revids = [None]
         for data in data:
+            if sys.platform == 'win32':
+                # time.time() has a precision of 1ms on Windows.
+                sleep(0.001)
             revids.append(self._dostore(oid, revids[-1], MinPO(data)))
         revids.reverse()
         del revids[-1]