bpo-26571: Reinit class variable _RUNNING

[williamnavaraj]

Initialising the class variable at the end of the object initialisation method. This will help to run this library smoothly in use cases such as in notebooks or GUI or other cell based or multiple turtle object instance execution. Otherwise it will raise Terminator error every second time. Will solve problems highlighted repeatedly in these stack overflow posts (With 1000s of views, current suggestions are some non-elegant workarounds such as importlib.reload(turtle) or or try-except and initializing again in the except as the _RUNNING becomes false at the end of last run or manually setting the _RUNNING flag). Logical to turn this to True after the initialization rather than during terminator exception. Change tested extensively and no issues found in any of the turtle-demo examples, notebooks or usual self-contained single instance execution.
https://stackoverflow.com/questions/45534458/python-turtle-terminator-even-after-using-exitonclick
https://stackoverflow.com/questions/34956937/weird-terminator-error-when-running-python-3-turtle-in-os-x
https://stackoverflow.com/questions/70252586/turtle-module-for-langtons-ant-in-python-giving-me-raise-terminator-turtle-ter
https://stackoverflow.com/questions/58078376/a-function-i-created-can-only-be-called-once-second-time-it-shows-an-error
https://stackoverflow.com/questions/54198108/how-can-i-close-a-window-after-using-turtle
https://stackoverflow.com/questions/67734319/running-turtle-multiple-times-but-receiving-a-terminator-error-after-the-first-r
https://stackoverflow.com/questions/43294665/turtle-done-not-working-in-spyder
https://stackoverflow.com/questions/50438762/python-turtle-window-crashes-every-2nd-time-running
https://stackoverflow.com/questions/41644324/how-to-close-python-turtle-properly
https://stackoverflow.com/questions/41548813/using-turtle-module-exitonclick/70552164
https://stackoverflow.com/questions/66750634/why-do-i-get-a-terminator-error-when-i-run-this-program
https://stackoverflow.com/questions/65546812/python-turtle-terminator
https://stackoverflow.com/questions/61963565/why-do-i-keep-getting-a-terminator-error-for-python-turtle

https://bugs.python.org/issue26571

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[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[williamnavaraj]

@serhiy-storchaka @ethanfurman PR became stale