Added Dutch National Flag algorithm #4636 (#4639)

* Added Dutch national flag sort Algorithm

* Changed file name to dnf_sort.py

* Added descriptive name and type hint 

Added descriptive name and type hint  for parameter with doctest for the function dnf_sort.

* Added test cases

* Added doctest cases

* Update sorts/dnf_sort.py

* Added doctest for dutch_national_flag_sort sorts/dnf_sort.py

* Update sorts/dnf_sort.py

* Added doctest for the function
dutch_national_flag_sort

* update file as per black code formatter

* Update dnf_sort.py

* Update and rename dnf_sort.py to dutch_national_flag_sort.py

Co-authored-by: Christian Clauss <cclauss@me.com>