Merge pull request matplotlib#13081 from tacaswell/doc_ban_pillow_540

DOC: forbid a buggy version of pillow for building docs

Author: dstansby

doc-requirements.txt

@@ -11,6 +11,6 @@ colorspacious
 ipython
 ipywidgets
 numpydoc>=0.8
-pillow>=3.4,<5.4
+pillow>=3.4,!=5.4.0
 sphinx-gallery>=0.2
 sphinx-copybutton