Fix setup.py requiring six module

[nmaludy]

For the last 2 days we've been seeing errors like this in our CI/CD pipelines:

    ----------------------------------------
ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.
    ERROR: Command errored out with exit status 1:
     command: /ci_stackstorm/packs/stackstorm-patching_encore/ci/virtualenv/bin/python -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/ci_stackstorm/packs/stackstorm-patching_encore/ci/virtualenv/src/st2sdk/setup.py'"'"'; __file__='"'"'/ci_stackstorm/packs/stackstorm-patching_encore/ci/virtualenv/src/st2sdk/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info
         cwd: /ci_stackstorm/packs/stackstorm-patching_encore/ci/virtualenv/src/st2sdk/
    Complete output (5 lines):
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/ci_stackstorm/packs/stackstorm-patching_encore/ci/virtualenv/src/st2sdk/setup.py", line 21, in <module>
        import six
    ImportError: No module named six
    ----------------------------------------

It appears to be caused by https://github.com/StackStorm/st2sdk/commit/6fdfe4b1f7de47571089ed6f6330fa772f7faec3#diff-b4ef698db8ca845e5845c4618278f29a

In setup.py we now import six and since six isn't necessarily installed in the virtualenv before installing st2sdk the installation fails.

This simple changes removes the dependency on six from setup.py so we should be able to install st2sdk like before.

@Kami can you check this out?

[Kami]

Good catch, thanks :+1: