get_dot_dependency_graph(): Give user control over stringification

This change removes the use of repr() to stringify statements. It replaces it with a user-controlled parameter which defaults to escaping only double quotes.

Motivation: Sometimes, it is useful to have labels that include escape sequences significant to the graphviz language. The string that comes from repr() doesn't allow us to do that because it escapes all the backslashes.

>>> print(str("a\lb"))
a\lb

>>> print(repr("a\lb"))
'a\\lb'