Bump sanic from 19.3.1 to 19.6.3 (#551)

Bumps [sanic](https://github.com/huge-success/sanic) from 19.3.1 to 19.6.3.
- [Release notes](https://github.com/huge-success/sanic/releases)
- [Changelog](https://github.com/huge-success/sanic/blob/master/CHANGELOG.rst)
- [Commits](sanic-org/sanic@19.03.1...v19.6.3)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Author: dependabot-preview[bot]

requirements_dev.txt

@@ -5,7 +5,7 @@ bumpversion==0.5.3  # pyup: update minor
 wheel==0.33.0  # pyup: update minor
 cryptography==2.7  # pyup: update minor
 psycopg2-binary==2.8.1  # pyup: update minor
-sanic==19.3.1;python_version>="3.6"  # pyup: update minor
+sanic==19.6.3;python_version>="3.6"  # pyup: update minor
 aiohttp==3.6.0;python_full_version>="3.5.3"  # pyup: update minor
 tornado==6.0  # pyup: update minor
 async_generator==1.10  # pyup: update minor