Update sphinx-gallery from 0.11.1 to 0.20.0

Author: pyup-bot

docs/requirements.txt

@@ -6,4 +6,4 @@ numpydoc==1.1.0
 sphinx==4.3.1
 sphinxcontrib-bibtex==2.4.1
 sphinxawesome-theme==3.2.1
-sphinx-gallery==0.11.1
+sphinx-gallery==0.20.0