Txaio

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- IMPORTANT: beginning release v20.1.1, we only support Python 3.5 or later
- new: moved ``time_ns`` and ``perf_counter_ns`` helper functions here

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- IMPORTANT: release v18.8.1 is the last release supporting Python 2. We will support Python 3.5 and later beginning with release v20.1.1.
- add API to support cancellation; this means passing a 1-argument callable to ``create_future`` and ``txaio.cancel`` to actually cancel a future
- support Python 3.7 (CI / testing added)

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- move to calver
- deprecate Python 3.3 support and CI testing

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- the asyncio version of ``make_logger`` now deduces a proper namespace instead of using the root (thanks `spr0cketeer <https://github.com/spr0cketeer>`_)

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- March 2, 2018

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- September 4, 2017
- fix: no longer install LICENSE file into installation directory (conflicts!)