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The app engine standard environment still offers Python 2.7. As there is a "python3-memcached 1.51" Python3 port for pure python memcached client library, [1] there is no reason to suppose that the upgrade to Python 3 will miss it. If your question refers to another environment, or situation, you are most welcome to elaborate. Which libraries do you refer to, when you write you are unable to use their latest version?  

[1] https://pypi.python.org/pypi/python3-memcached/1.51

Hi,

  So we are currently running a Standard AppEngine application (python 2.7) which uses the AppEngine memcache to store some session variables and user fingerprints. One of our use cases is doing PDF reports, for which we are currently using WeasyPrint (which has performance issues on Python 2 and the current version only supports Python 3), since we cannot install Python 3 on Standard AppEngine (and migrating to the flexible environment would require a lot of code rewriting as AppEngine Memcache would not work) we are running a small VM with a WSGI server to generate the reports.
  This instance must have access to the same AppEngine Memcache so we can verify fingerprints and authenticate the user on a different domain so using python3-memcached is not an option as far as I understand. Or is it possible to access the AppEngine memcache with python3-memcached?

Thanks for your support,

We need a little more time to investigate.

Meanwhile, seen that you have your own machine in GCE, the small VM with a WSGI server, you may try the python3-memcached 1.51 library, to connect to Memcache.

Once Python 3 is available in the standard environment, you should be able to use your pdf reporting software, and use memcache as well. There is no reason to assume that, suddenly, memcache will not be any more available.
Meanwhile, if you are into compute engine solutions, you may consider implementing a readily available image, that would offer you memcached services till the standard environment gets upgraded to Python 3. This is "Memcached Certified by Bitnami". You can inspect this offer by choosing "launcher" in the left-frame drop down menu, and then searching for "memcached" in the search window on top.

> Once Python 3 is available in the standard environment, you should be able to use your pdf reporting software, and use memcache as well. There is no reason to assume that, suddenly, memcache will not be any more available.

→ https://cloud.google.com/appengine/docs/standard/python3/python-differences
> The Memcache service is not available in Python 3. If you need access to a hosted caching service, you can use a third party memcache service like Redis Labs Cloud.

Can we re-open this issue? If not I'll likely open one that specifically mentions "Python 3 Standard: Memcache"

I stand corrected: Memcache is not available in Python 3. In fact, version 3.7 has just been released and it is in beta stage. You may find Memcache libraries for Python 3 on the web, so things may change. Engineering has been made aware of your feature request, and will address it in due course. No estimated time to implementation has been set. Meanwhile, you may follow developments in this thread.