Emcache

--------------------
* Dropped Python 2 and 3.6 support
`321 <https://github.com/pinterest/pymemcache/pull/321>`_
`363 <https://github.com/pinterest/pymemcache/pull/363>`_
* Begin adding typing
* Add pluggable compression serde
`407 <https://github.com/pinterest/pymemcache/pull/407>`_

--------------------
* Handle blank ``STAT`` values.

--------------------
* ``Client.get`` returns the default when using ``ignore_exc`` and if memcached
is unavailable
* Added ``noreply`` support to ``HashClient.flush_all``.

--------------------
* Sockets are now closed on ``MemcacheUnexpectedCloseError``.
* Added support for TCP keepalive for client sockets on Linux platforms.
* Added retrying mechanisms by wrapping clients.

--------------------
* Idle connections will be removed from the pool after ``pool_idle_timeout``.

--------------------
* Fix ``HashClient.{get,set}_many()`` with UNIX sockets.