Add pyproject.toml tests for some options

[WilliamJamieson]

In astropy/astropy#15247 (comment), it was requested that pytest-doctestplus add unit testing to confirm the support of doctest_nocursedirs and doctest_subpackage_requires. This PR adds the requested tests.

Note that this PR requires bumping the minimum pytest version from 4.6 to 6. While this is a big jump it is inline with the Scientific Python Style Guide. In particular PP302 says that projects should use pytest 6 or later. Moreover, the method to test pyproject.toml files for pytest configuration, makepyprojecttoml, was only added in pytest 6.0.0.

I added skips on the test cases involving the pyproject.toml.

[pllim]

Thanks! Though I want Simon to have a chance to review since he originally brought up the concern.

[bsipocz]

Now that it doesn't bump the requirements (that would definitely be a blocker), it looks all good to me. Thanks!

[pllim]

Thanks, all!

[saimn]

Thanks for adding the tests @WilliamJamieson.