Define self._handle up-front to prevent warning in __del__

Since initialization failures happen (usually) in `get_handle`, the
instance would not have a proper `self._handle` defined and fail on
`__del__` which checks for this field's value.

Author: filmor

clr_loader/hostfxr.py

@@ -13,6 +13,8 @@
 
 class DotnetCoreRuntime(Runtime):
     def __init__(self, runtime_config: Path, dotnet_root: Path, **params: str):
+        self._handle = None
+
         if _IS_SHUTDOWN:
             raise RuntimeError("Runtime can not be reinitialized")