Bump Sphinx version from 2.0 to 7.x (#47)

This fixes the documentation build. I looked at the documentation
output and everything seems fine. Note that the most recent Sphinx
version is 8.x, but that requires Python >= 3.10.

Bumping to 7.x resolves CI issues for now without requiring a larger
Python update of the whole code base.

Author: ldionne

tox.ini

@@ -43,7 +43,7 @@ commands =
 [testenv:docs]
 skip_install = true
 deps =
-    sphinx<2.0.0
+    sphinx==7
     sphinx_bootstrap_theme
 
 commands =