Added slither into python book (#3552)

free-programming-books.md

@@ -2261,6 +2261,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Python Programming](https://upload.wikimedia.org/wikipedia/commons/9/91/Python_Programming.pdf) - Wikibooks (PDF) (2.6)
 * [Scipy Lecture Notes](http://scipy-lectures.github.io)
 * [SICP in Python](http://www-inst.eecs.berkeley.edu/~cs61a/sp12/book/) (3.2)
+* [Slither into Python: An introduction to Python for beginners](https://www.slitherintopython.com) (3.7)
 * [Suporting Python 3: An In-Depth Guide](http://python3porting.com) (2.6 - 2.x & 3.1 - 3.x)
 * [Test-Driven Web Development with Python](http://www.obeythetestinggoat.com/pages/book.html) - Harry Percival (3.3 - 3.x)
 * [Text Processing in Python](http://gnosis.cx/TPiP/) - David Mertz (2.3 - 2.x)