Simplified the answer to exercise 3 in python essentials (#688)

* Changed the implementation of exercise 3 in python essentials to make it
faster, more pythonic and use techniques explained in the text

* Reincorporated the original solution as the implementation is easy to
understand.

The short solution has been left as an alternative.

Author: jez-w

source/rst/python_essentials.rst

@@ -1050,14 +1050,24 @@ Here's one solution:
 .. code-block:: python3
 
     def f(string):
-        count = 0
-        for letter in string:
-            if letter == letter.upper() and letter.isalpha():
-                count += 1
-        return count
+    count = 0
+    for letter in string:
+        if letter == letter.upper() and letter.isalpha():
+            count += 1
+    return count
     f('The Rain in Spain')
 
 
+An alternative, more pythonic solution, would be:
+
+.. code-block:: python3
+
+    def count_uppercase_chars(s):
+        return sum([c.isupper() for c in s])
+
+    count_uppercase_chars('The Rain in Spain')
+
+
 Exercise 4
 ----------