docs: use recommonmark as an extension

- source_parsers deprecated in sphinx 3.0
- Since sphinx 1.4, it can (should) be used as a direct extension:
  https://github.com/readthedocs/recommonmark/pull/43

docs/source/conf.py

@@ -20,6 +20,7 @@ extensions = [
     'autodoc_traits',
     'sphinx_copybutton',
     'sphinx-jsonschema',
+    'recommonmark',
 ]
 
 templates_path = ['_templates']
@@ -66,8 +67,6 @@ def setup(app):
     app.add_transform(AutoStructify)
 
 
-source_parsers = {'.md': 'recommonmark.parser.CommonMarkParser'}
-
 source_suffix = ['.rst', '.md']
 # source_encoding = 'utf-8-sig'