Fix |release| RST substitution in docs having incomplete values for development versions

[RonnyPfannschmidt]

@RonnyPfannschmidt this results in the |release| RST substitution having the value of 8.2 which might have a weird perception in changelog draft titles: https://docs.pytest.org/en/8.2.x/changelog.html#to-be-included-in-vrelease-if-present / https://docs.pytest.org/en/latest/changelog.html#to-be-included-in-vrelease-if-present.

Should this be something like +dev in the end but without a variable component, something static? Though, perhaps in RTD it'd be okay to use the long original version.

Ursprünglich gepostet von @webknjaz in #12467 (comment)

[webknjaz]

I filed an upstream issue on letting these not influence the caching mechanism: sphinx-doc/sphinx#12480.