Update unregister implementation to remove all references to the previously registered function

[cygann]

When pyterminate unregisters a function, not all references to that function get cleared. This causes problems when the registered function holds references to objects that are expected to be deallocated after that function gets unregistered from pyterminate.

This updates the unregister implementation to:

• Remove the exit handler wrapper of func from the _func_to_wrapper dictionary
• If func is the actively registered signal handler, register the previous signal handler instead. In any case, for any registered signal handler whose previous handler was func, update the previous handler to func's previous.
• Remove func from the _signal_to_prev_handlerdictionary

[jeremyephron]

@cygann Thanks! I've added some changes to your fork to get it ready to merge. The implementation changes can be seen in 9d74749.

Essentially, your original implementation did not properly handle signal unregistration in a couple of ways (trying to register None, not unregistering if there was a default handler, ...). A bit more of the code needed to adjust to accommodate that. Additionally, it didn't handle proper deletion of references during the normal exit path where the signal or exit handler runs. The changes I added address that.

Importantly, I've also added tests to ensure this works and doesn't regress.

For some reason, Python 3.7 behaves a bit strangely (caught in the tests). I don't necessarily think this is worth the time to look into right now, so I've dropped support for Python 3.6 and 3.7, which seems appropriate given Ubuntu 20.04 is on Python 3.8.

I think this is an important improvement to the package. Merging.