Icontract

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* Encapsulated adding contracts for integrators (202)
* Added support for error-as-instance (201)
* Added support for coroutine (197)
* Added support for async (196)

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* Removed automatic registration with Hypothesis and replaced it with a hook that
downstream libraries such as icontract-hypothesis can use (181)
* Refactored and added tests for integrators (182)

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* Integrated with icontract-hypothesis (179)
* Refactored for icontract-hypothesis (178)
* Added special arguments `_ARGS` and `_KWARGS` (176)
* Tested with typeguard (175)
* Tested with `dataclasses.dataclass` (173)
* Added invariants to namedtuple (172)
* Added support for recomputation of f-strings (170)
* Exempted `__new__` from invariant checks (168)
* Added support for named expressions in contracts (166)

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* Acted upon deprecation warning ins ``ast`` module when generating the
violation error message.

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* Denormalized icontract_meta so that icontract can be installed on
readthedocs.

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* Disabled invariant checks during the construction to avoid attribute errors
on uninitialized attributes