Port tests from pytest-describe to normal pytest tests

[stsewd]

The pytest-describe project seems abandoned, isn't compatible with a current version of pytest. We should port this tests. and delete the pytest-describe dependency to update pytest to a more current version.

Raised in https://github.com/rtfd/readthedocs.org/pull/4242/files#r195538134

[mashrikt]

Can we replace describe-blocks with class?
Like: def describe_validate_use_system_site_packages():
to: class ValidateUseSystemSitePackages:

[stsewd]

Yes, I was thinking the same, this issue is kind of blocked till we end the v2 of the configuration file https://github.com/orgs/rtfd/projects/2 (not sure if you can see the project).

[mashrikt]

I've made a PR for this, since I was working on it.
I understand it has to be merged later. Would still appreciate your review!

[ericholscher]

Fixed in #4429