Issue #19912: Fixed numerous bugs in ntpath.splitunc().

* splitunc() no more return illegal result for paths with redundant slashes.
* splitunc() now correctly processes the 'İ' character
  (U+0130, LATIN CAPITAL LETTER I WITH DOT ABOVE).
* Deprecation warnings now emitted for every use of splitunc().
* Added tests for splitunc().

diff --cc Lib/ntpath.py
Simple merge

diff --cc Lib/test/test_ntpath.py
Simple merge

diff --cc Misc/NEWS
Simple merge