Enforce visibility of deprecation warnings

Originally reported by: jaraco (Bitbucket: jaraco, GitHub: jaraco)

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With the release of 3.0, it became apparent that the DeprecationWarning for #65 wasn't adequate to raise awareness of the pending removal.

Setuptools should probably enforce visibility of DeprecationWarnings in the setuptools and pkg_resources namespaces.

Something like

warnings.filterwarnings("always", category=DeprecationWarning, module="setuptools\.|pkg_resources\.")

I'm not quite sure yet where this would happen. I'm considering it could happen at import time or possibly only during select commands initialization.

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• Bitbucket: https://bitbucket.org/pypa/setuptools/issue/159

[pganssle]

I think the way most people solve this (matplotlib, dateutil, pandas, etc) is to just not use DeprecationWarning at all. Instead, you create a subclass of Warning, like so:

class PkgResourcesDeprecationWarning(Warning):
    """Class for warning about deprecations in PkgResources"""

That way, they are on by default, but they're still easy to filter out.

[martonp]

@robinjhuang and I would like to work on this during the GA python sprint