Waterfall dialogs/tests aren't compatible with Python 3.8

[v-kydela]

Version

4.7

Describe the bug

Two waterfall tests are failing when using Python 3.8

To Reproduce

Steps to reproduce the behavior:

1. Install Python 3.8
2. Build Bot Builder Python and run the tests according to these instructions: https://github.com/microsoft/botbuilder-python/wiki/building-the-sdk
3. See failing tests

Expected behavior

Bot Builder Python should be compatible with the latest version of Python

[bug]