pymemcache support

[zzzeek]

an old PR that never got completed is at https://bitbucket.org/zzzeek/dogpile.cache/pull-requests/34/this-gives-initial-support-for-pymemcache/diff, but also there seems to be more interest in this backend: https://review.openstack.org/#/c/616304/ .

[zzzeek]

I've adapted the PR at https://gerrit.sqlalchemy.org/#/c/sqlalchemy/dogpile.cache/+/947/

[zzzeek]

cc @morganfainberg , needs a few more tweaks

[zzzeek]

there seems to be no interest in this right now. contributors feel free to reopen + implement.

[zzzeek]

OK we have a PR at #197

[sqla-tester]

Moisés Guimarães de Medeiros has proposed a fix for this issue in the master branch:

Add pymemcache backend https://gerrit.sqlalchemy.org/c/sqlalchemy/dogpile.cache/+/2494