Group internal resource lists and reorganize xprocess into a package

[northernSage]

Addressing #67

• group semantically related lists used for resource cleanup
• create a new class XProcessResources to hold all process resources
• update XProcess.ensure to use XProcessResources
• add __del__ to XProcessResources so resources are automatically released on end of test execution
• only explicitly clean up through XProcess._force_resource_cleanup upon interruptions
• turn xprocess into a package and use find_packages in order to avoid issues such as _internal.py isn't shipped with the 0.18.0 version installed by PyPi #77
• add changelog

[RonnyPfannschmidt]

the new structure is not a enhancmenet

resource should be a list of a simple classes that contain all resources of a process, just moving attributes into dict keys is not a sensible change, - literally all this does it make the code to access it longer

[northernSage]

I recall taking some time to figure out all allocated resources when we had the resource leak problem back in #61. It would have been a lot quicker if all resources (Popen instances, XProcessInfo instances and file handles) were all neatly and explicitly grouped in a resources structure such as a dict. So in my opinion this is still an improvement.

That being said, I agree that keeping the resources in a separate class seems like a better way of going about this. If I understand correctly, you're proposing having something like XProcessResources where all resources of a given XProcess instance would be kept (?). If this is the case, we could even further separate concerns by moving the cleanup logic from XProcess._clean_up_resources to XProcessResources.free. @RonnyPfannschmidt

[RonnyPfannschmidt]

Structurally I would recommend a context manager or a close method to match other common apis

[northernSage]

We now have a XProcessResources class where all resources used by a running process will be kept. Each XProcessResources knows how to clean itself up by using its release method and will do so upon exit under normal circumstances through its destructor __del__. Upon any interruptions during the test run, the clean-up process will be forced by calling XProcess._force_clean_up in our InterruptionHandler. When you find the time, some feedback on this would be great @RonnyPfannschmidt

[RonnyPfannschmidt]

Unfortunately I may not be able to get back to to this this weekend