Use pre-commit for CI tests, add black hook (#2163)

* WIP precommit on top level

* update CI

* circleci fixes

* intentionally fail black

* use --show-diff-on-failure in CI

* fix command order

* rebreak a file

* apply black

* run on whole repo

.circleci/config.yml

             pip install --upgrade setuptools
             cd ml-agents-envs && pip install -e .
             cd ../ml-agents && pip install -e .
-            pip install black pytest-cov==2.6.1 codacy-coverage==1.3.11
+            pip install pre-commit pytest-cov==2.6.1 codacy-coverage==1.3.11
             cd ../gym-unity && pip install -e .
 
       - save_cache:
           name: Check Code Style for ml-agents and gym_unity using black
           command: |
             . venv/bin/activate
-            black --check ml-agents
-            black --check ml-agents-envs
-            black --check gym-unity
+            pre-commit run --show-diff-on-failure --all-files
 
       - run:
           name: Verify there are no hidden/missing metafiles.

utils/validate_meta_files.py

                 num_matched += 1
 
     if unmatched:
-        raise Exception(f"Mismatch between expected files and their .meta files: {sorted(unmatched)}")
+        raise Exception(
+            f"Mismatch between expected files and their .meta files: {sorted(unmatched)}"
+        )
-    main()
+    main()

.pre-commit-config.yaml

+exclude: '^$'
+fail_fast: false
+repos:
+-   repo: https://github.com/python/black
+    rev: '19.3b0'
+    hooks:
+    -   id: black