Property-cached

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* Catch SyntaxError from asyncio import on older versions of Python, thanks to asottile

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* Really fixed tests, thanks to pydanny

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* Added conftest.py to manifest so tests work properly off the tarball, thanks to dotlambda
* Ensured new asyncio tests didn't break Python 2.7 builds on Debian, thanks to pydanny
* Code formatting via black, thanks to pydanny and ambv

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* Added asyncio support, thanks to vbraun
* Remove Python 2.6 support, whose end of life was 5 years ago, thanks to pydanny

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* Validate for Python 3.6

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* Drop some non-ASCII characters from HISTORY.rst, thanks to AdamWill
* Added official support for Python 3.5, thanks to pydanny and audreyr
* Removed confusingly placed lock from example, thanks to ionelmc
* Corrected invalidation cache documentation, thanks to proofit404
* Updated to latest Travis-CI environment, thanks to audreyr