Added function for Leetcoode problem Length of Longest Substring.

[und1n3]

Description

Added funcion for a Leetcode problem, Length of Longest Substring (https://leetcode.com/problems/longest-substring-without-repeating-characters/) in Python.

Type of change

• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the style guidelines(Clean Code) of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings

[und1n3]

Is this PR eligible to the Hacktoberfest? Thanks!