Before reporting a bug, please make sure you have the latest cachetools version installed:

pip install --upgrade cachetools

Describe the bug
For extending a class, the docs say all cache implementations call popitem() to evict items from the cache and indicate that overriding the popitem() can catch expiration from any class.

This does not work with TTLCache.

Expected result
Overriding popitem() should be called when an item is expired.

Actual result
The popitem() method is not called.

Reproduction steps

import time

from cachetools import TTLCache


class MyCache(TTLCache):
    def popitem(self):
        key, value = super().popitem()
        print('Key "%s" evicted with value "%s"' % (key, value))
        return key, value


c = MyCache(maxsize=20, ttl=2)
c["a"] = 1
time.sleep(1)
c["b"] = 2
time.sleep(4)
c["c"] = 3