OpenStack requirements

[hemna]

The next release of openstack is going to require python3.6 and python 3.7 support.
We cannot add pyxcli as a library to the global-requirements due to lacking py37 support.

Also, pyxcli doesn't pass pep8 tests currently either.

pep8 run-test: commands[0] | flake8 pyxcli
pyxcli/client.py:206:34: F821 undefined name 'basestring'
        if isinstance(endpoints, basestring):
                                 ^
pyxcli/client.py:226:34: F821 undefined name 'basestring'
        if isinstance(endpoints, basestring):
                                 ^
pyxcli/errors.py:134:1: E302 expected 2 blank lines, found 1
@CommandExecutionError.register("VOLUME_IS_MASTER")
^
pyxcli/errors.py:138:1: E302 expected 2 blank lines, found 1
@CommandExecutionError.register("PARTIAL_SUCCESS")
^
pyxcli/errors.py:718:1: E302 expected 2 blank lines, found 0
@CommandExecutionError.register("LOGIN_FAILURE_USER_FAILED_TO_LOGIN",
^
pyxcli/examples.py:14:10: E211 whitespace before '('
    print (vol.name)
         ^
pyxcli/examples.py:19:10: E211 whitespace before '('
    print (pool.name)
         ^
pyxcli/pool.py:92:49: W504 line break after binary operator
            if (not entry.client.is_connected() or
                                                ^
pyxcli/pool.py:94:21: E117 over-indented
                    xlog.debug("XCLIClientPool: clearing stale client %s",
                    ^
pyxcli/response.py:106:29: F821 undefined name 'basestring'
        if isinstance(item, basestring):
                            ^
pyxcli/response.py:108:37: F821 undefined name 'long'
        elif isinstance(item, (int, long)):
                                    ^
pyxcli/transports.py:310:34: F821 undefined name 'basestring'
        if isinstance(endpoints, basestring):
                                 ^
pyxcli/helpers/xml_util.py:52:24: F821 undefined name 'basestring'
    if isinstance(obj, basestring) or not use_repr:
                       ^
pyxcli/mirroring/cg_recovery_manager.py:151:65: W504 line break after binary operator
                     "being cloned." % (cg_id, snap_group_name) +
                                                                ^
pyxcli/mirroring/cg_recovery_manager.py:168:65: W504 line break after binary operator
                     "being cloned." % (cg_id, snap_group_name) +
                                                                ^
ERROR: InvocationError for command /home/waboring/devel/3rdparty/pyxcli/.tox/pep8/bin/flake8 pyxcli (exited with code 1)

[hemna]

Also, Openstack requires that all libraries work in python 3.6 and python3.7.

https://review.opendev.org/#/c/658104/