PdbMeta: fix instantiation of Pdb subclasses

[bretello]

PdbMeta meta is the metaclass responsible for reusing existing Pdb instances in order to persist state across different breakpoint()/set_trace invocations

When Pdb is subclasses, this causes Pdbpp-specific parameters to be passed to the class' init, causing instantiation to fail.

fixes #60

[bretello]

This is currently causing testing/test_pdb.py::test_config_gets_start_filename to fail, because Pdbpp instances aren't being reused across different set_trace invocations