Drop support for older Pytest versions

[edgarrmondragon]

I got some numbers from the downloads dataset on BigQuery:

version	downloads (past month)	percent	last release
4.x	1,446,865	1.1%	2020-06-05
5.x	2,464,986	1.9%	2020-06-02
6.x	9,514,327	7.3%	2021-08-30
7.x	49,503,260	38.1%	2023-12-31
8.x	66,480,236	51.1%	2024-04-27

It's probably worth at least dropping 4.x and 5.x. Curious what you think @henryiii.

[henryiii]

I’ve already dropped 4 and 5 locally. :)