build: remove `sphinx<9` pin in `docs.txt` when `sphinx_rtd_theme` supports it

[cadenmyers13]

Problem

sphinx_rtd_theme doesn't support sphinx>=9

Proposed solution

When it does support it. remove the pin from docs.txt

[cadenmyers13]

@sbillinge Ah, i think this is the pin you were talking about

[sbillinge]

there it is! So we should test, but I think we can remove it.