Bump Sphinx version to the same as Trio

TeamSpen210 · TeamSpen210 · commit 5bf059eff183 · 2023-08-06T11:07:59.000+10:00
This is needed to have :value: for :py:data:, to show what Maybe equals
diff --git a/ci/rtd-requirements.txt b/ci/rtd-requirements.txt
@@ -1,4 +1,4 @@
 # RTD is currently installing 1.5.3, which has a bug in :lineno-match:
-sphinx >= 1.6.1
+sphinx >= 4.0, < 6.2
 sphinx_rtd_theme
 sphinxcontrib-trio
