feat: drop support for Python 3.8

[finswimmer]

Python 3.8 will be EOL on October. Let's drop the support than for running Poetry itself with it.

[TheSven73]

From py3.9 onwards, we can make some executor.py code simpler:

            if self._shutdown:
                # Cancelling further tasks from being executed
                [task.cancel() for task in tasks]
                self._executor.shutdown(wait=True)

=>

            if self._shutdown:
                self._executor.shutdown(wait=True, cancel_futures=True)

[github-actions[bot]]

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