Crash in `ClassDerivedObject.tp_dealloc` when object is collected by cycle collector

Pythonnet version: 3.0.0.dev1
Repro:
```python
from Python.Test import BaseClass

class Derived(BaseClass):
  __namespace__ = 'Test'

inst = Derived()
cycle = [inst]
del inst
cycle.append(cycle)
del cycle

import gc
gc.collect() # crash here: in ClassDerivedObject.tp_dealloc because ClassBase.tp_clear nulled tpHandle
```
