Fix race-condition in .items() and .values()

[kormat]

In python3.5, a race-condition in both .items() and .values() can be triggered. Here's an example script:

import expiringdict
import time

d = expiringdict.ExpiringDict(10, 1)
d[1] = 1
time.sleep(0.5)
d[2] = 2
time.sleep(0.7)
print(d.items())

Here's what the race-condition looks like:

Traceback (most recent call last):
  File "./test.py", line 11, in <module>
    print(d.items())
  File "/home/kormat/expiringdict/__init__.py", line 115, in items
    for key in self:
RuntimeError: OrderedDict mutated during iteration

The fix is very simple - make a list of the keys before iterating.

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This change is

[horkhe]

Fixed in #28