Handle asynctest not working in Python 3.8

[bachya]

Describe what the PR does:

This PR fixes the fact that asynctest no longer works in Python 3.8. The PR still ensures that it can be used for Python 3.6 and 3.7.

Does this fix a specific issue?

N/A

Checklist:

• Confirm that one or more new tests are written for the new functionality.
• Update README.md with any new documentation.
• Add yourself to AUTHORS.md.

[sourcery-ai]

Sourcery Code Quality Report (beta)

Merging this PR leaves code quality unchanged.

Quality metrics	Before	After	Change
Complexity	0.00	0.00	0.00
Method Length	90.07	90.07	0.00
Quality	8.04	8.04	0.00

Other metrics	Before	After	Change
Lines	166	166	0

Changed files	Quality Before	Quality After	Quality Change
tests/test_websocket.py	8.04	8.04	0.00

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Overall	Recommendation
tests/test_websocket.py	test_data_async	0	183.84	6.37	Split out functionality
tests/test_websocket.py	test_data_sync	0	183.84	6.37	Split out functionality

Please see our documentation here for details on how these metrics are calculated.

We are actively working on this report - lots more documentation and extra metrics to come!

Let us know what you think of it by mentioning @sourcery-ai in a comment.

[codecov]

Codecov Report

Merging #72 into dev will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               dev       #72   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          144       144           
=========================================
  Hits           144       144           

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