Testrail-api

Updated endpoints:
* add_milestone
* get_users (thank Enacero )

Added endpoints:
* add_run_to_plan_entry
* update_run_in_plan_entry
* delete_run_from_plan_entry

* Casting a boolean to an integer for get requests

* Updated docstrings for TestRail 6.3
* Added argument exc_iterations
* Updated tests

* Fixed invalid annotation to `results.add_result_for_case` bug 34

* Updated developer dependencies
* Added Python 3.9 for tests
* Fix type annotation ( thank tylervick )
* Added retry argument