BLD: update pillow dependency

Pillow has a security issue for <6.2.0 (CVE-2019-16865).

This is in violation of our normal support window for dependencies,
however we are making an exception due to the CVE.

Author: tacaswell

requirements/doc/doc-requirements.txt

@@ -12,6 +12,5 @@ colorspacious
 ipython
 ipywidgets
 numpydoc>=0.8
-pillow>=3.4,!=5.4.0
 sphinx-gallery>=0.2
 sphinx-copybutton

setup.py

@@ -261,7 +261,7 @@ def run(self):
             "cycler>=0.10",
             "kiwisolver>=1.0.1",
             "numpy>=1.11",
-            "pillow",
+            "pillow>=6.2.0",
             "pyparsing>=2.0.1,!=2.0.4,!=2.1.2,!=2.1.6",
             "python-dateutil>=2.1",
         ],