Fix RTD builds (#1043)

Author: sloria

tox.ini

@@ -36,7 +36,7 @@ commands = mypy src/ {posargs}
 
 [testenv:docs]
 dependency_groups = docs
-commands = sphinx-build docs/ docs/_build {posargs}
+commands = sphinx-build docs/ {posargs:docs/_build}
 
 ; Below tasks are for development only (not run in CI)