Bump koa from 3.0.1 to 3.0.3 (#731)

Bumps [koa](https://github.com/koajs/koa) from 3.0.1 to 3.0.3.
- [Release notes](https://github.com/koajs/koa/releases)
- [Changelog](https://github.com/koajs/koa/blob/master/History.md)
- [Commits](koajs/koa@v3.0.1...v3.0.3)

---
updated-dependencies:
- dependency-name: koa
  dependency-version: 3.0.3
  dependency-type: direct:development
...

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