Replace import * with explicit imports, allowing linter to detect the missing imports reported in #53.

Update test to reflect the copy of the hash name stored because pypy doesn't implement the undocumented .name attribute on HASH objects.

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extra : rebase_source : 4f9aabd0efe87de76f85f91c4c379e48f68ffac4

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extra : rebase_source : 2f37f043a6cc87cc015efa318699bc09785dbd87

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extra : rebase_source : 4468b50706cd9ab6d0ae990ee6f785b314837dba

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extra : rebase_source : 549f10a7b864152871edccf3d0f9e0bf57fb8bf0

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extra : rebase_source : e6f4e62a87e24933c8821fdef141d194b90874d3

Make certain no previous imports of old versions of pkg_resources are floating around before importing the newly installed setuptools.

Load version from the file rather than by importing the module (which is subject to variance based on sys.path). Fixes #43.

Use 'is_valid' instead of simply 'valid' or 'check', which are less clear about the purpose of the method. Fixes AttributeError introduces in 0.9.2. Fixes #42.