Bump singer-sdk from 0.3.18 to 0.4.0

Bumps [singer-sdk](https://gitlab.com/meltano/sdk) from 0.3.18 to 0.4.0.
- [Release notes](https://gitlab.com/meltano/sdk/tags)
- [Changelog](https://gitlab.com/meltano/sdk/blob/master/CHANGELOG.md)
- [Commits](https://gitlab.com/meltano/sdk/compare/v0.3.18...v0.4.0)

---
updated-dependencies:
- dependency-name: singer-sdk
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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