Update docs requirements for Python 3.9

Author: hugovk

docs/requirements.txt

@@ -1,6 +1,6 @@
 mistune
 jsonschema
 ipython
-sphinx<4
-jinja2<3
-markupsafe<2
+sphinx
+jinja2
+markupsafe

docs/source/conf.py

@@ -58,9 +58,9 @@
 # Undocumented trick: if we def setup here in conf.py, it gets called just
 # like an extension's setup function.
 def setup(app):
-    app.add_javascript("show-code.js")
-    app.add_javascript("facebox.js")
-    app.add_stylesheet("facebox.css")
+    app.add_js_file("show-code.js")
+    app.add_js_file("facebox.js")
+    app.add_css_file("facebox.css")
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']