Fix tests for Python3.6 by removing asyncio-mode as it's not supported.

[iulianmac]

What:

CI tests are currently failing for Python3.6 with the error:

ERROR: usage: __main__.py [options] [file_or_dir] [file_or_dir] [...]
__main__.py: error: unrecognized arguments: --asyncio-mode=auto
  inifile: None

Why:

CI/unit tests are failing for Python3.6.
I believe this failure was introduced with this change and pytest-asyncio seems to be 3.7+ compatible.

How:

Do not run pytest with --asyncio-mode=auto for Python3.6

Risks:

None identified.

Checklist:

• Added tests, if you've added code that should be tested (N/A)
• Updated the documentation, if you've changed APIs( N/A)
• Ensured the test suite passes
• Made sure your code lints
• Completed the Contributor License Agreement ("CLA")

[coveralls]

Pull Request Test Coverage Report for Build 2165206971

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.145%

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Totals
Change from base Build 1945670569:	0%
Covered Lines:	2637
Relevant Lines:	2801

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💛 - Coveralls

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