fix: Fixed default threadpool count for pyver> 3.9

[gavin-aguiar]

Description

From python 3.9 onwards, the threadpool count is not set, worker won't be putting a limit on the max_workers count in the created threadpool.

Fixes #

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[vrdmr]

Thanks for doing this. qq - why wasn't it caught before in the tests - where the expectation is that the number of threads default should not have been 1 in the first place