executor as ApiHandle class attribute

[jtpio]

Follow-up to #93

Trying the latest locally, there seems to be a couple of warnings related to having the __del__ method:

Exception ignored in: <function ApiHandler.__del__ at 0x7ff024d01040>
Traceback (most recent call last):
  File "/path/to/miniforge3/envs/jupyter-resource-usage-release/lib/python3.9/site-packages/jupyter_resource_usage/api.py", line 23, in __del__
    super().__del__()
AttributeError: 'super' object has no attribute '__del__'

Looks like it should be fine to keep the previous behavior of having the executor as a class attribute.

cc @fcollonval

[fcollonval]

Sorry for that one - usually I prefer to have thread shutdown explicitly controlled. But I'm fine with the rollback.

[jtpio]

No problem, thanks for checking.

I think we can cut a new 0.6.0 release with this change and #93. And then potentially improve the handling of the executor separately.