Import error because of requests_async

[chenjr0719]

Describe the bug
Import error after install Sanic. The requests_async does not be included in setup.py's install_requires. All tests can be passed because the requests_async is installed by tox.

Code snippet
Related source in testing.py:
https://github.com/huge-success/sanic/blob/e36f398aa689cb1852a171f9e6e60f78d8fa21d5/sanic/testing.py#L1-L4

Related source in setup.py:
https://github.com/huge-success/sanic/blob/e36f398aa689cb1852a171f9e6e60f78d8fa21d5/setup.py#L78-L85

Expected behavior
Import without any error.

Environment (please complete the following information):

• OS: Ubuntu 18.04
• Version master(e36f398)

Additional context

Two possible fixes:

1. Add requests_async in setup.py's install_requires.
2. As the old implementation, only import in _local_request().
   https://github.com/huge-success/sanic/blob/ef6d78c580ed4f3410e029ed514d2d5ee3380ce3/sanic/testing.py#L13-L20

[ahopkins]

Yes, I noticed this the other day. There's a discussion about it on the forums? I'll push a fix using strategy 1 you outlined.

[harshanarayana]

@chenjr0719 @ahopkins Working on a PR for this and #1591 together. Give me a few hours.

[ahopkins]

@harshanarayana I was just about to..., but I will hold off on it and just push another little change that has been annoying me.