Setting special limits for Python 3.[6|7]

[vrdmr]

Description

For Python 3.6 and 3.7, setting max workers to large values is problematic as the implementation of ThreadPoolExecutor for both that versions do not reuse threads until max_workers number is hit, and that is a danger to the functioning of the app. This PR limits the number of max_workers to 32 for 3.6 and 3.7 and allows it to be sys.maxint for Python 3.8+.

Reference doc - ThreadPoolExecutor

Changed in version 3.8: Default value of max_workers is changed to min(32, os.cpu_count() + 4). This default value preserves at least 5 workers for I/O bound tasks. It utilizes at most 32 CPU cores for CPU bound tasks which release the GIL. And it avoids using very large resources implicitly on many-core machines.

ThreadPoolExecutor now reuses idle worker threads before starting max_workers worker threads too.

Fixes #873

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[AnatoliB]

LGTM