Pin asyncpg version to 0.31

[BaurzhanSakhariev]

https://github.com/MagicStack/asyncpg/releases/tag/v0.32.0 causes failure of test_result_streaming_using_fetch_size

It passes locally with 0.3.1

[BaurzhanSakhariev]

From https://jenkins.crate.io/blue/organizations/jenkins/CrateDB%2Fqa%2Fcrate_qa/detail/crate_qa/1793/pipeline/44/

raise exceptions.NoActiveSQLTransactionError(

    'cursor cannot be created outside of a transaction')

and then

asyncpg.exceptions.PostgresSyntaxError: line 1:1: extraneous input 'ROLLBACK' expecting {<

I think this is related to MagicStack/asyncpg#1350

So open question is to

1. Find out why sending BEGIN causes failures.

2. Maybe implement ROLLBACK as a no-op? Support PostgreSQL's ROLLBACK as a NOOP crate#11608

   We already closed add support for the ROLLBACK statement crate#8351 because we thought it can lead to unexpected issues.

   On the other hand, we already support START TRANSACTION (accept start transaction statements crate#11323) and BEGIN/COMMIT(add support for BEGIN and COMMIT crate#7431),
   so it's a bit odd (or at least unexpected for users) that we don't support their counterpart as a no-op.

[matriv]

thx, please fix typo ping -> pin.