{CI} Remove usage of six

[jiasli]

Description

Thanks to @fengzhou-msft for the finding.

Previously, six is installed by packaging 20.4.

> pipdeptree -p six -r
six==1.14.0
  - packaging==20.4 [requires: six]
    - pytest==6.1.1 [requires: packaging]
      - azdev==0.1.28 [requires: pytest>=5.0.0]
      - azure-cli-testsdk==0.2.4 [requires: pytest]
      - pytest-forked==1.3.0 [requires: pytest>=3.10]
        - pytest-xdist==2.1.0 [requires: pytest-forked]
          - azdev==0.1.28 [requires: pytest-xdist]
      - pytest-xdist==2.1.0 [requires: pytest>=6.0.0]
        - azdev==0.1.28 [requires: pytest-xdist]
    - tox==3.20.1 [requires: packaging>=14]
      - azdev==0.1.28 [requires: tox]

Now six has been dropped since packaging 20.5 (pypa/packaging#331), causing CI to fail.

https://dev.azure.com/azure-sdk/public/_build/results?buildId=635666&view=logs&jobId=4cf87b3f-d609-5b69-870a-4ac948acfd42&j=4ac732a2-b9e4-5fde-3ff2-aac0b888c429&t=7defb86d-ca26-5ebc-39fe-803ea5c317dd

==================================== ERRORS ====================================
_____ ERROR collecting azure/cli/telemetry/tests/test_telemetry_client.py ______
ImportError while importing test module '/home/vsts/work/1/s/src/azure-cli-telemetry/azure/cli/telemetry/tests/test_telemetry_client.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/opt/hostedtoolcache/Python/3.8.6/x64/lib/python3.8/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
azure/cli/telemetry/tests/test_telemetry_client.py:22: in <module>
    from azure.cli.telemetry.components.telemetry_client import (CliTelemetryClient, _NoRetrySender, http_client_t)
azure/cli/telemetry/components/telemetry_client.py:8: in <module>
    import six
E   ModuleNotFoundError: No module named 'six'
=========================== short test summary info ============================
ERROR azure/cli/telemetry/tests/test_telemetry_client.py
!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!
=============================== 1 error in 0.25s ===============================
ERROR: InvocationError for command /home/vsts/work/1/s/src/azure-cli-telemetry/.tox/py38/bin/pytest (exited with code 2)
___________________________________ summary ____________________________________
ERROR:   py38: commands failed

This PR fixes the CI by removing the usage of six as Azure CLI has dropped Python 2 support.