[Feature Request] Support for Pytest 6

[rbpatt2019]

Is your feature request related to a problem? Please describe.
Currently, I use the dash-testing extra for writing tests for my apps. Big fan of how streamlined it makes the progress! However, it pegs pytest at v5.3.5 as a requirement, which means I can't take advantage of the new Pytest v6 release - can you say pyproject.toml and full type hinting!

Describe the solution you'd like
Support for pytest v6

Describe alternatives you've considered
Not supporting pytest v6. But there are some pretty powerful features added here, so (personally) I don't think this is a good decision.

Additional context
I'm not sure if this would be a big undertaking or not. I'm happy to tackle it, if people are interested, though I'll probably have questions along the way!

[alexcjohnson]

Thanks @rbpatt2019 - we'd be happy to use the latest and greatest, and I don't know what kind of breaking changes pytest v6 makes but AFAIK we're not using really esoteric features of it anyway. So feel free to make a PR and maybe everything will just work - CI will tell us :)

[rbpatt2019]

See #1368.