Icontract

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* Exempted ``__init__`` from inheritance of preconditions and postconditions if defined in the
concrete class.

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* Contracts are observed and inherited with property getters, setters and deleters.
* Weakining of preconditions of a base function without any preconditions raises ``TypeError``.
* ``__getattribute__``, ``__setattr__` and ``__delattr__`` are exempted from invariants.
* Slot wrappers are properly handled.
* Fixed representation of conditions with attributes in generator expressions
* Added reference to sphinx-contract

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* Added reference to ``pyicontract-lint`` in the README
* Made ``inv`` a class

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* Added support for class and static methods

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* Fixed different signatures of ``DBCMeta`` depending on Python version (<=3.5 and >3.5) due to differing signatures
of ``__new__`` in ``abc.ABCMeta``

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* Removed dependency on ``meta`` package and replaced it with re-parsing the file containing the condition
to represent the comprehensions