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 =