API: `warnings.warn` instead of `print` if `dot_parser` fails to import
API: `warnings.warn` instead of `print` if `dot_parser` fails to import

because this warning will appear once only anyway (upon import),
thus the default `warnings` behavior of printing only once each warning is similar.
Compatibility with `pyparsing >= 1.5.7` implies that this warning shouldn't appear.
pydot/pydot,erocarrera/pydot