Required pytest version is broken on Python 3.10

[prryplatypus]

Describe the bug
The current version of pytest required by Sanic (5.2.1) is incompatible with Python 3.10 and throws an exception when running tests.

Expected behavior
Tests run on Python 3.10.

Additional context
A fix was released with pytest version 6.2.4. See pytest's related issue: pytest-dev/pytest#8539.

[sjsadowski]

PEP 619 has the target date for 3.10 is 2021-10-04 (Today) so I imagine we should fix this in 21.9.X as well as for the LTS

[ahopkins]

Release is @ 17:00 UTC

After that, we will get #2257 running with whatever needed changes, and then a bump in supported versions as @sjsadowski mentioned.

[sjsadowski]

@ahopkins quick note the GH runner seems to be interpreting "3.10" as "3.1" per the test failure here: https://github.com/sanic-org/sanic/pull/2257/checks?check_run_id=3777968217

[ahopkins]

@ahopkins quick note the GH runner seems to be interpreting "3.10" as "3.1" per the test failure here: https://github.com/sanic-org/sanic/pull/2257/checks?check_run_id=3777968217

I know. I saw that. I believe it should be using 3.10-dev. But, did not dig into it further since the release is imminent and I imagine the regular release version is not far behind.