Revert DOC/CI: temp pin matplotlib for doc build #20045 (#20704)

TomAugspurger · web-flow · commit 0bd8a5a62b78 · 2018-04-15T13:57:27.000-05:00
diff --git a/ci/requirements-3.6_DOC.run b/ci/requirements-3.6_DOC.run
@@ -5,7 +5,7 @@ sphinx
 nbconvert
 nbformat
 notebook
-matplotlib=2.1*
+matplotlib
 seaborn
 scipy
 lxml
