Remove Python pins to use current version

Author: hugovk

.pre-commit-config.yaml

@@ -5,7 +5,6 @@ repos:
     hooks:
       - id: black
         args: [--safe, --quiet]
-        language_version: python3.6
   - repo: https://github.com/pre-commit/pre-commit-hooks
     rev: v2.3.0
     hooks:
@@ -19,4 +18,3 @@ repos:
         files: ^(CHANGELOG.rst|README.rst|HOWTORELEASE.rst|changelog/.*)$
         language: python
         additional_dependencies: [pygments, restructuredtext_lint]
-        python_version: python3.6