Default isolation level None breaks Transaction.__repr__

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• asyncpg version: 0.22.0
• PostgreSQL version: 13.2
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?: No, I do not.
• Python version: 3.9.1
• Platform: Ubuntu 20.10
• Do you use pgbouncer?: No
• Did you install asyncpg with pip?: Yes
• If you built asyncpg locally, which version of Cython did you use?: N/A
• Can the issue be reproduced under both asyncio and
  uvloop?: Yes

Since Transaction.__repl__ does ' '.join(attrs), and attrs contains self._isolation, it will fail with a TypeError with the new default isolation level of None.

Regression introduced in #622