Mockcache for python3

First release of mockcache3, forked from mockcache.

• Python 3 compatible.
• Support any iterable in get_multi method.
• Bug fix: deep copy object when it is added to cache to avoid changes in the original object.
• Validate keys for all methods to work like original memcached.
• Upgraded all old toolchain with poetry, pytest, black, flake8, isort, tox and others.