py(deps[docs]): Pin sphinx<9 to avoid breaking changes

why: Sphinx 9.x introduced breaking changes that affect sphinxarg/ext.py
(mock() function moved to internal _dynamic package) and mypy fails on
PEP 695 type statement syntax. Sphinx 9 also drops Python 3.10/3.11 support
(requires Python 3.12+).
what:
- Add sphinx<9 constraint to dev and docs dependency groups
- Update uv.lock to downgrade from 9.1.0 to 8.1.3

Author: tony

pyproject.toml

@@ -52,7 +52,7 @@ Changes = "https://github.com/tmux-python/libtmux/blob/master/CHANGES"
 [dependency-groups]
 dev = [
   # Docs
-  "sphinx",
+  "sphinx<9",
   "furo",
   "gp-libs",
   "sphinx-autobuild",
@@ -81,7 +81,7 @@ dev = [
 ]
 
 docs = [
-  "sphinx",
+  "sphinx<9",
   "furo",
   "gp-libs",
   "sphinx-autobuild",