MNT: Use version check instead of except for pytest>=7

[pllim]

This is a direct follow-up of #157. When checking for pytest>=7 becomes easy, we need to replace this code:

https://github.com/astropy/pytest-doctestplus/blob/afc77eb62bf5dce848a872ff2207107d8744db77/pytest_doctestplus/plugin.py#L470-L471