Bump sanic from 20.3.0 to 20.12.6 in /requirements

Bumps [sanic](https://github.com/sanic-org/sanic) from 20.3.0 to 20.12.6.
- [Release notes](https://github.com/sanic-org/sanic/releases)
- [Changelog](https://github.com/sanic-org/sanic/blob/main/CHANGELOG.rst)
- [Commits](sanic-org/sanic@v20.3.0...v20.12.6)

---
updated-dependencies:
- dependency-name: sanic
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements/dev.txt

@@ -1,5 +1,5 @@
 aioredis==1.3.1
 asyncpg==0.21.0
-sanic==20.3.0
+sanic==20.12.6
 flake8==3.8.1
 isort==4.3.21

requirements/prod.txt

@@ -1,3 +1,3 @@
 aioredis==1.3.1
 asyncpg==0.21.0
-sanic==20.3.0
+sanic==20.12.6