psf/black code formatting (#1421)

* added sol3.py for problem_20

* added sol4.py for problem_06

* ran `black .` on `\Python`

dynamic_programming/fractional_knapsack.py

@@ -20,6 +20,7 @@ def fracKnapsack(vl, wt, W, n):
         else sum(vl[:k])
     )
 
+
 if __name__ == "__main__":
     import doctest