TST: bump minimum pillow to 6.2.1 because it has wheels for py38

We need wheels to get webp support (which we test).

Author: tacaswell

requirements/testing/minver.txt

@@ -4,7 +4,7 @@ cycler==0.10
 kiwisolver==1.0.1
 numpy==1.19.0
 packaging==20.0
-pillow==6.2.0
+pillow==6.2.1
 pyparsing==2.2.1
 python-dateutil==2.7
 fonttools==4.22.0