Add 'allow-unknown-extras'

Also add the required changes to zc.recipe.egg.

Fixes #457

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fix #295: temporary egg files are created as read only and can't be deleted with shutil on Windows.

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Compare using Distribution.parsed_version to account for normalization.

'python -m zc.buildout.tests TestEasyInstall'

Fixes #451.

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Use the new PyPI at https://pypi.org/.

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Fix for #442 issue

`pkg_resources.SetuptoolsVersion` got deprecated in `setuptools` `v39.0.0`.
Moved on using stable and recommended `pkg_resources.parse_version`
function that does the same. For more details on the topic
see http://setuptools.readthedocs.io/en/latest/history.html#v39-0-0.

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Correctly detect upgrade with extension that depends on setuptools

With an extension (like mr.developer) that depends on setuptools *and* with
the new update-the-workingset-immediately functionality introduced in 2.11,
the old have-we-been-upgraded-ourselves functionality didn't function reliably
anymore.

Clarify intent plus spelling/grammar fixes

Highlight names uniformly

Fix a typo, minor grammar issues

properly close resources

Fixes ``buildout.py:574: ResourceWarning: unclosed file <_io.TextIOWrapper...>```