Bump pgproto version

[Atem18]

• asyncpg version: 0.21.0
• PostgreSQL version: 12
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?: PostgreSQL in container
• Python version: 3.8.6
• Platform: Linux
• Do you use pgbouncer?: no
• Did you install asyncpg with pip?: yes
• If you built asyncpg locally, which version of Cython did you use?: no
• Can the issue be reproduced under both asyncio and
  uvloop?: think so

Hi,

I have an error when using Pyinstaller with asyncpg which correspond to : MagicStack/py-pgproto#8

I can see that pgproto https://github.com/MagicStack/asyncpg/tree/master/asyncpg is from the 2 May but I would like to have the version of 29 May.

Can you bump it please ? Or I can make a pull request.

Thanks a lot.