Products.btreefolder2

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- More PEP8 compliance.

- Add icon for Bootstrap ZMI.

- Avoid deprecation warnings by using current API.

- Add support for Python 3.7.

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- added tox configuration

- Python 3 compatibility

- Update to require and be compatible with Zope 4.

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- Update Dependencies (no ZODB, but BTrees)

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- Require ExtensionClass >= 4.1a1 (Compatible w/ Zope 4).

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- Added case for clean-up routine where the meta type index contains
keys that are not in the tree.

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- Provide security declaration for `BTreeFolder2Base.hasObject` method.

- Add some tests for correct `getattr` behavior.

- Minor `__getattr__` and `_getOb` optimizations.