Update thirt.py

kyu_6/a_rule_of_divisibility_by_13/thirt.py:1 at module level:
        D205: 1 blank line required between summary line and description (found 0)

        Multi-line docstrings consist of a summary line just like a one-line
        docstring, followed by a blank line, followed by a more elaborate
        description. The summary line may be used by automatic indexing tools;
        it is important that it fits on one line and is separated from the
        rest of the docstring by a blank line.

kyu_6/a_rule_of_divisibility_by_13/thirt.py:1 at module level:
        D400: First line should end with a period (not '3')

        The [first line of a] docstring is a phrase ending in a period.

kyu_6/a_rule_of_divisibility_by_13/thirt.py:11 in public function `thirt`:
        D205: 1 blank line required between summary line and description (found 0)

        Multi-line docstrings consist of a summary line just like a one-line
        docstring, followed by a blank line, followed by a more elaborate
        description. The summary line may be used by automatic indexing tools;
        it is important that it fits on one line and is separated from the
        rest of the docstring by a blank line.

kyu_6/a_rule_of_divisibility_by_13/thirt.py:11 in public function `thirt`:
        D400: First line should end with a period (not 's')

        The [first line of a] docstring is a phrase ending in a period.

kyu_6/a_rule_of_divisibility_by_13/thirt.py:11 in public function `thirt`:
        D401: First line should be in imperative mood; try rephrasing (found 'The')

        [Docstring] prescribes the function or method's effect as a command:
        ("Do this", "Return that"), not as a description; e.g. don't write
        "Returns the pathname ...".

kyu_6/rotate_the_letters_of_each_element/group_cities.py:10 in public function `group_cities`:
        D401: First line should be in imperative mood (perhaps 'Group', not 'Grouping')

        [Docstring] prescribes the function or method's effect as a command:
        ("Do this", "Return that"), not as a description; e.g. don't write
        "Returns the pathname ...".

kyu_6/a_rule_of_divisibility_by_13/thirt.py

@@ -1,5 +1,6 @@
 """
-Solution for -> A Rule of Divisibility by 13
+Solution for -> A Rule of Divisibility by 13.
+
 Created by Egor Kostan.
 GitHub: https://github.com/ikostan
 """
@@ -9,6 +10,8 @@
 
 def thirt(n: int) -> int:
     """
+    Rule of Divisibility by 13.
+
     The function which processes this sequence of operations
     on an integer n (>=0). `thirt` will return the stationary number.
     :param n: int