Fix airflow db shell needing an extra keypress to exit

[renzepost]

Related: #26186
As @blag noted in #26186, pressing Ctrl-D (EOF) to exit the interactive shell requires an additional keypress to actually exit.
It seems that select.select() was blocking, and setting the timeout parameter to 0 fixes the issue of the interactive shell not exiting immediately after pressing Ctrl-D (or typing \q + Enter in psql). The docs for select state:

When the timeout argument is omitted the function blocks until at least one file descriptor is ready. A time-out value of zero specifies a poll and never blocks.

I tested the fix with all the supported db backends and saw no adverse effects, but hopefully someone with more experience with select can tell if this is a reasonable approach.

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[potiuk]

Fantastic! Thanks @renzepost !