Icontract

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* Replaced ``typing.Type`` with ``type`` so that icontract works with Python 3.5.2

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* Added ``error`` argument to the contracts

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* Removed ``ast_graph`` module which was only used for debugging
* Prefixed internal modules with an underscore (``_represent`` and ``_recompute``)

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* ``recompute`` propagates to children of generator expressions and comprehensions
* Optimized parsing of condition lambdas by considering only lines local to the decorator

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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