setup.py: Force Sphinx > 2 for readthedocs

FIX

readthedocs uses an old version of sphinx for projects created before
2020:
mgeier/sphinx-last-updated-by-git#1

So force a newer version.

setup.py

@@ -77,9 +77,9 @@ def make_console_script(name):
 }
 
 extras_require["doc"] = [
-    "sphinx >= 1.8",
     # Force ReadTheDocs to use a recent version, rather than the defaults used
     # for old projects.
+    "sphinx > 2",
     "sphinx_rtd_theme >= 0.5.2",
     "sphinxcontrib-plantuml",
     "nbsphinx",