Add pytest deselect by variable

[martin-kokos]

EPYTEST_DESELECT appears to be popular, but there is no mention of it, so adding it to the guide.

[mgorny]

Your grep-foo failed. It's in the section about epytest because — obviously — it works for pytest only.

[martin-kokos]

I don't understand. If I come across this page, I will start sed'ding away tests while I should go looking into pytest_utils to find out how to deselect them.

[mgorny]

There's a big "Note" about it just below what you added. Perhaps change it to make the option names more explicit, and link to the detailed doc in pytest chapter.