Bump asyncpg from 0.18.3 to 0.21.0 in /py (#339)

Bumps [asyncpg](https://github.com/MagicStack/asyncpg) from 0.18.3 to 0.21.0.
- [Release notes](https://github.com/MagicStack/asyncpg/releases)
- [Commits](MagicStack/asyncpg@v0.18.3...v0.21.0)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

py/requirements.txt

@@ -4,7 +4,7 @@ aiohttp==3.7.4
 aiohttp-session==2.7.0
 aioredis==1.3.0
 arq==0.15
-asyncpg==0.18.3
+asyncpg==0.21.0
 async-timeout==3.0.1
 bcrypt==3.1.6
 buildpg==0.1