Let downstream library to define causes object

[notatallshaw]

Addresses comments made in #93 (comment)

Provider now has a causes method that allows it to define what the causes object looks like.

The returned object must implement the following:

• Have truthiness
• Be copyiable using copy.copy

For example if would be possible to implement the method like so to preserve the existing behavior (would work on Python 2.7 to 3.10):

def causes(self, causes):
    return [i for c in causes for i in c.information]

There is an accompanying pip PR (pypa/pip#10732) that leverages this new interface to resolve the performance issues described in pypa/pip#10621

Need some hints / help defining a better type than "Any" for "causes" if that matters.

[notatallshaw]

There are some architectural differences between the way I've implemented this and how Requirement and Candidate work.

@uranusjr if you get a chance some time let me know if this is what you were thinking or if I missed the shot and need to make some correction.

[notatallshaw]

Upon some further thinking this approach is type unsafe, I need to study how other objects are implemented a bit better first.