show color in hook outputs when attached to a tty

testing/resources/stdout_stderr_repo/.pre-commit-hooks.yaml

@@ -1,4 +1,8 @@
 -   id: stdout-stderr
     name: stdout-stderr
     language: script
-    entry: ./entry
+    entry: ./stdout-stderr-entry
+-   id: tty-check
+    name: tty-check
+    language: script
+    entry: ./tty-check-entry

testing/resources/stdout_stderr_repo/tty-check-entry

@@ -0,0 +1,12 @@
+#!/usr/bin/env python
+import sys
+
+
+def main():
+    print('stdin: {}'.format(sys.stdin.isatty()))
+    print('stdout: {}'.format(sys.stdout.isatty()))
+    print('stderr: {}'.format(sys.stderr.isatty()))
+
+
+if __name__ == '__main__':
+    exit(main())