[watchmedo] Avoid zombie sub-processes when running shell-command without --wait (#897)

* [watchmedo] Avoid zombie sub-processes when running shell-command without --wait

* lint

* reference the issue rather than PR in the changelog

* avoid shlex.join which was added in Python 3.8

* increase test wait time

* try further increasing wait time to get tests passing on Windows

* fix is_process_running()

* still debugging on Windows...

* apparently Windows doesn't like shell-quoted Python executables
4 files changed