This adds a simple recovery path in case an SSL connection receives
an SSL_WANT_READ or WRITE error.  Either error can occur while
reading or writing.  The error indicates that the underlying
operation should be retried after the socket is once again readable
or writable (per the error code).

Closes #618

Co-authored-by: James E. Blair <jeblair@redhat.com>

Feat/build coverage

chore(core): Update matrix: add pypy3, drop py27

Update .travis.yml definition per warnings.
Tests are now only run on py37, py38 and pypy3.
Bumped Zookeeper to 3.5.8.

This is a faster and more idiomatic way of using itertools.chain.
Instead of computing all the items in the iterable and storing them
in memory, they are computed one-by-one and never stored as a huge
list. This can save on both runtime and memory space.

Test Kazoo with CPython 2.7, CPython 3.7 and CPython 3.8. Deploy new Kazoo version only on CPython 3.8 job success.

As of Python 3.8, "is" with a literal is a syntax warning because of the
confusion between equality and instance identity it represents.

Issue #607

Let's make our life a little easier using those templates.

See https://help.github.com/en/github/building-a-strong-community/about-issue-and-pull-request-templates for reference.

feat(core): interoperate with Go client

Per https://github.com/houndci/hound/issues/1769

Allows configurable multi-implementations cooperations in locks (e.g.
Zookeeper python & go clients contending for the same lock).

fix(core): sync() return should be unchrooted

Resolves #601

feat(test): Move to pytest

With this patch, requests issued while the client is in the
'CONNECTING' state get queued instead of raising a misleading
'SessionExpiredError'.

This fixes https://github.com/python-zk/kazoo/issues/374, and brings
Kazoo more in line with the Java and C clients.

See the 'kazoo.client.KazooClient.state' documentation as well as
these discussions for more details:

https://github.com/python-zk/kazoo/pull/570#issuecomment-554798550
https://github.com/python-zk/kazoo/pull/583#issuecomment-586422386

Satisfy new Hound style/lint checks
Upgrade to latest 3.5.6/3.4.14 Zookeeper releases.

Since krb5 1.3 due to switch to Xenial.

fix(tests): prefix with toxinidir variable with a / to ensure path to ensure-zookeeper-env.sh is correct

 * Install debian packages for KDC as part of Travis init.
 * Setup a loopback mini KDC for running tests.
 * Run SASL tests as part of Travis builds.
 * Improve harness cluster to support:
    * Reconfiguration when environment changes.
    * Different JAAS configurations (DIGEST/GSSAPI).
 * Moved SASL tests into own module, with specially configured harness.
 * Bumped default timeout to 15 sec to mitigate false negatives on
   Travis.