Tuberd

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- Fix enum class serialization. The str_enum helper now produces enum.Enum()
classes in Python that are also str() subclasses. For all 3 serializers we
use (json, orjson, cbor2) this is enough to correctly serialize enum classes.

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- CI/CD improvements: builds wheels for manylinux and OS X

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- A "simple" (non-asyncio) client now lives alongside the asyncio client. The
points of entry for these are "resolve" and "resolve_simple". They are
approximately API-compatible, but the non-asyncio version entirely avoids
async code style (async def / await / ...).

- Multiple calls (using a Context or SimpleContext) can now optionally continue
after errors, using the continue_on_error flag. When this is True, all calls
in a Context/SimpleContext run to completion even if some of them fail.
The default behaviour remains the same - when continue_on_error is False,
server-side execution proceeds in sequence and halts at the first call that
encounters an error.

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- Headers are now properly included in binary builds (.whl files)
- Corrected PYTHONPATH for "make test"; broken in 0.11.

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- resolve now required (20)
- tuber package rearranged to tuber.server / tuber.client; registry moved to server (19)
- linter (black) added to CI/CD flow (17)
- client-side Python packages (aiohttpd) no longer required in server environment (18)
- setuptools now builds client and server (14)

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- Adds support for CBOR as a transport mechanism.