resolve segfault in dng observed with Python 3.9

Calling the PythonInterpreter constructor set up some Python objects via
Boost Python before Python was properly initialized. This caused a segfault
in _PyObject_GC_Malloc in Python. Not sure what finally triggered that
segfault. Different Boost version? Different Python version? Different
compiler? Bruce Lee beating up Chuck Norris? https://youtu.be/HFm2L03AetU