Log Message:
py-graphviz: updated to 0.12

Version 0.12

Fix missing escaping of backslashes, e.g. in labels (pull request DNGros).

Add quiet argument to standalone view() function, and quiet_view
argument on .render() and .view() methods. Supresses the stderr
output of started viewer processes (unavailable on Windows).

Add basic debug logging via the stdlib logging module.

Reformatted some examples, improved tests by using autospec for mocks.