Upgrade to python 3.6+

[bernd-wechner]

As requested:

2. hacked on it
3. sending pull request

Used f-strings so Python 3.6+

Gave it a spiun and worthadding a help option on the CLI (which I haven't done) and worth noting in the code that a server of format "unix:" works fine for a server (I happen to use mine that way).

Also probably worth adding a key dumper akin to memcdump provided with memcached.