Aiotools

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* Python 3.8 is now officially supported.
* **server:** Fix errors when `multiprocessing.set_start_method("spawn")` is used.
- NOTE: This is now the default for macOS since Python 3.8.
- KNOWN ISSUE: [12](https://github.com/achimnol/aiotools/issues/12)
* Remove some packaging hacks in `__init__.py` and let setuptools read the version
from a separate `aiotools/VERSION` text file.

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* **context:** Fix `aclosing()`'s `__aexit__()` exception arguments.

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* **context**, **server:** Catch asyncio.CancelledError along with BaseException to
make the cancellation behavior consistent in Python 3.6, 3.7, and 3.8.

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* **server:** Fix yields of the received stop signal in main/worker context managers
when using threaded workers.

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* **server:** Updated stop signal handling and now user-defined worker/main context
managers have a way to distinguish the stop signal received. See the updated
docs for more details.

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* This ia a technical release to fix a test case preventing the automated CI
release procedure.