Use Python 3.6 for Black (aws#2)

This prevents Black from using Python 3.7+ only syntax when we still support Python 3.6

Author: adamchainz

.pre-commit-config.yaml

@@ -3,5 +3,5 @@ repos:
     rev: 19.3b0
     hooks:
       - id: black
-        language_version: python3.7
-        exclude_types: ['markdown', 'ini', 'toml', 'rst']
+        language_version: python3.6
+        exclude_types: ['markdown', 'ini', 'toml', 'rst']