Update black and work around an isort bug (#4242)

requirements-tests-py3.txt

@@ -1,6 +1,6 @@
 git+https://github.com/python/mypy.git@master
 typed-ast>=1.0.4
-black==19.3b0
+black==19.10b0
 flake8==3.8.2
 flake8-bugbear==20.1.4
 flake8-pyi==20.5.0

stdlib/3/encodings/__init__.pyi

@@ -1,3 +1,6 @@
+# This comment serves as a workaround for timothycrosley/isort#1027 and can
+# be removed when a release with a fix was released.
+
 import codecs
 
 from typing import Any