Asyncio-redis

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- Added support for Python 3.8
- Dropped support for Python <3.6
- Deprecated 'loop' parameter in Connection.create and Pool.create
- Added support for zpopmin and zrangebylex methods
- Fix DeprecationWarning for inspect.formatargspec()
- Fix missing object asyncio.streams.IncompleteReadError
- Intersphinx: all classes that can be imported from the top-level module must now be
referenced from the top-level module in the docstrings too.
E.g. :class:`asyncio_redis.exceptions.Error` has changed to
:class:`asyncio_redis.Error`.
- Added flake8, black, and isort as mandatory CI checks

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- Added Python 3.7 compatibility.
- Added several options to zadd and zincrby.
- Bugfix in concurrency with transactions:

* Transactions were apparently prone to race conditions, because crutial
parts of the transaction did a 'yield' while entering the transaction in
the 'multi' command. This has been fixed now by using an asyncio.Lock.
* Because of this, it also became possible to start multiple
transactions or commands on the same connection. If the lock was
acquired by another transaction/command, will will wait instead of
raising an exception.

- Fall back to asyncio.async when asyncio.ensure_future is not available.
- Encode scripts using utf-8 instead of ascii.
- Better handle timeouts during pubsub.

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Fixes:
- Better handle timeouts during pubsub. (Handle IncompleteReadError.)
- Use asyncio.ensure_future instead of deprecated async() when available.
- Encode scripts using utf-8 instead of ascii.
- Bugfix in concurrency with transactions:
* Transactions were apparently prone to race conditions, because crutial
parts of the transaction did a 'yield' while entering the transaction in
the 'multi' command. This has been fixed now by using an asyncio.Lock.
* Because of this, it also became possible to start multiple
transactions or commands on the same connection. If the lock was acquired
by another transaction/command, will will wait instead of raising an
exception.

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Fixes:
- Added a few missing asyncio.coroutine decorators for Python 3.5
compatibility.

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Fixes:
- Correctly handle empty chunks returned by the 'scan' function.
- Correctly handle returned values in `spop`.
- Fixed default '*' argument for scan operations. (When BytesEncore is used.)

New features:
- Add a RedisProtocol.watch() method
- Added LIMIT support for ZRangeByScore
- Added count parameter to 'scan' cursors.

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Fixes:
- Rpoplpush can return nil when the list is empty.