Fix `test_seqscans` by disabling statement timeout.
Also, replace increasing statement timeout with disabling it for
performance tests. This should make tests more stable and allow us to
observe performance degradation instead of test failures.
Newer version of mypy fixes buggy error when trying to update only boto3 stubs.
However it brings new checks and starts to yell when we index into
cusror.fetchone without checking for None first. So this introduces a wrapper
to simplify quering for scalar values. I tried to use cursor_factory connection
argument but without success. There can be a better way to do that,
but this looks the simplest
More rows, and test with serial and parallel plans. But fewer iterations,
so that the tests run in < 1 minutes, and we don't need to mark them as
"slow".
