Scripts with model converting don't exit properly

[fmazzasc]

As shown in the example, scripts for converting and dumping hummingbird models don't exit properly.
For Ubuntu and centOS scripts remain stuck after the last line while for macOS the stack trace is:

Fatal Python error: take_gil: PyMUTEX_LOCK(gil->mutex) failed
Python runtime state: finalizing (tstate=0x7fceb1409790)

Abort trap: 6

This is something that we might need to understand in the future