Tests: sphinxdev tox environment installs Sphinx from master (#1575)

It seems we had a bug in the name and it wasn't installing Sphinx from `master`.
I expect this test to fail now because we pin `Sphinx<8`, but that's fine for now.

Author: humitos

tox.ini

@@ -24,7 +24,7 @@ deps =
     sphinx71: Sphinx>=7.1,<7.2
     sphinx72: Sphinx>=7.2,<7.3
     sphinxlatest: Sphinx
-    dev: https://github.com/sphinx-doc/sphinx/archive/refs/heads/master.zip
+    sphinxdev: https://github.com/sphinx-doc/sphinx/archive/refs/heads/master.zip
 allowlist_externals =
     echo
 commands =