Switch to new FindPython3 cmake command

Summary: switched to new FindPython3 cmake command since old version caused problems on windows when with non-anaconda python distributions

Test Plan: tested by current continuous integration. will cause build errors if it does not work correctly.

Reviewers: O1 MITK Core, kislinsk

Reviewed By: O1 MITK Core, kislinsk

Subscribers: kislinsk

Maniphest Tasks: T25424

Differential Revision: https://phabricator.mitk.org/D206