MemcachedCache backend get_many does not work on Python 3

[Tim Graham]

These two test failures are fixed with the attached patch. Should this be backported to 1.5.x?

======================================================================
ERROR: test_cache_versioning_get_set_many (cache.tests.MemcachedCacheTests)
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Traceback (most recent call last):
  File "/home/tim/code/django/tests/cache/tests.py", line 685, in test_cache_versioning_get_set_many
    self.assertEqual(self.cache.get_many(['ford1','arthur1']),
  File "/home/tim/code/django/django/core/cache/backends/memcached.py", line 95, in get_many
    _[m[k]] = v
KeyError: ':1:arthur1'

======================================================================
ERROR: test_get_many (cache.tests.MemcachedCacheTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/tim/code/django/tests/cache/tests.py", line 234, in test_get_many
    self.assertEqual(self.cache.get_many(['a', 'c', 'd']), {'a' : 'a', 'c' : 'c', 'd' : 'd'})
  File "/home/tim/code/django/django/core/cache/backends/memcached.py", line 95, in get_many
    _[m[k]] = v
KeyError: ':1:a'

[Aymeric Augustin]

I'd rather replace list(map(lambda ...))) with a list comprehension.

Otherwise, yes, please commit and backport.

[Tim Graham <timograham@…>]

In 3c736207a37840109f16f004c343c77b5ee389b6:

Fixed #20722 -- Fixed MemcachedCache backend get_many on Python 3.

[Tim Graham <timograham@…>]

In 7788c7189036bcd78692aaae1dd4260d7a7563bf:

[1.6.x] Fixed #20722 -- Fixed MemcachedCache backend get_many on Python 3.

Backport of 3c736207a3 from master

[Tim Graham <timograham@…>]

In 7bbf6971329d7fdd5343895c7e8c292f7e64d95e:

[1.5.x] Fixed #20722 -- Fixed MemcachedCache backend get_many on Python 3.

Backport of 3c736207a3 from master