Auto-format all files using yapf and pre-commit (#1173)

Automatic fixes produced by yapf formatting using `--style=google`, as
well as common pre-commit checks such as trailing whitespace removal,
double quote fixer, and newlines at the end of a file.

.pre-commit-config.yaml

@@ -9,10 +9,12 @@ repos:
     hooks:
     -   id: trailing-whitespace
     -   id: end-of-file-fixer
+    -   id: double-quote-string-fixer
     -   id: check-yaml
     -   id: check-docstring-first
     -   id: name-tests-test
     -   id: requirements-txt-fixer
+    -   id: check-merge-conflict
 
 -   repo: https://github.com/pre-commit/mirrors-yapf
     rev: v0.29.0