Move conditional RTD dependencies to 'docs' extra

docs/conf.py

@@ -16,8 +16,6 @@ import sys
 import os
 import re
 
-on_rtd = os.getenv('READTHEDOCS') == 'True'
-
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.
@@ -36,15 +34,11 @@ extensions = [
     'sphinx.ext.autodoc',
     'sphinx.ext.extlinks',
     'sphinx.ext.intersphinx',
+    'sphinx.ext.napoleon',
     'sphinxcontrib.asyncio',
     'details'
 ]
 
-if on_rtd:
-  extensions.append('sphinxcontrib.napoleon')
-else:
-  extensions.append('sphinx.ext.napoleon')
-
 autodoc_member_order = 'bysource'
 
 extlinks = {