Add comment to clarify sphinx-argparse version pinning

requirements-dev.txt

@@ -4,6 +4,8 @@
 # Newer sphinx needed for proper type hint support in docstrings
 sphinx>=3.0.0
 sphinxcontrib-napoleon>=0.5.0
+# sphinx-argparse 0.4.0 is the last version to support Python 3.9
+# see https://sphinx-argparse.readthedocs.io/en/latest/changelog.html#id3
 sphinx-argparse==0.4.0
 sphinx-book-theme<=1.1.2,>=1.0.0
 sphinx-copybutton==0.5.2