Fix confused usage of input in the Python tutorial

From the context it looks like this was previously comparing
Python 2's `raw_input` and `input` functions, and that this was
missed during the upgrade to Python 3. Drop the comparison and
explicitly parse the inputs to numbers to let the reader focus
on the task at hand (computing the average).

Fixes #533

Author: PeterJCLaw

tutorials/python.md

@@ -112,11 +112,12 @@ Exercises: Variables and Mathematics
 
 ### Average calculator ###
 
-The first two lines of this program put two numbers entered by the user into variables `a` and `b`. (The `input` function is like `input`, but returns a number (e.g. `42`) when you enter one, rather than a string (like `"42"`).) Replace the comment with code that averages the numbers and puts them in a variable called `average`.
+The first two lines of this program put two numbers entered by the user into variables `a` and `b`.
+Replace the comment with code that averages the numbers and puts them in a variable called `average`.
 
 ~~~~~ python
-a = input("Enter first number: ")
-b = input("Enter second number: ")
+a = float(input("Enter first number: "))
+b = float(input("Enter second number: "))
 
 # Store the average of a and b in the variable `average`