Added Problem Set (#4792)

* Update problem-sets-competitive-programming.md

Added Practice Python to Problem Sets

* Update problem-sets-competitive-programming.md

problem-sets-competitive-programming.md

@@ -90,6 +90,7 @@
 * [PEG Judge](http://wcipeg.com)
 * [PKU Online Judge](http://poj.org)
 * [Ponder This!](https://www.research.ibm.com/haifa/ponderthis/index.shtml)
+* [Practice Python](https://www.practicepython.org)
 * [ProblemBook.NET](https://github.com/AndreyAkinshin/ProblemBook.NET)
 * [Project Euler](https://projecteuler.net)
 * [Python Practice Projects](http://pythonpracticeprojects.com)