Bump asyncpg from 0.20.1 to 0.21.0 (#40)

Bumps [asyncpg](https://github.com/MagicStack/asyncpg) from 0.20.1 to 0.21.0.
- [Release notes](https://github.com/MagicStack/asyncpg/releases)
- [Commits](MagicStack/asyncpg@v0.20.1...v0.21.0)

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Author: dependabot-preview[bot]