Add missing syntax highlighting to python.md (#2154)

Fixes missing code highlighting in half of the Python code snippets.

The top half of the page had code examples formatted as code blocks with
4 space character indentations and no syntax highlighting as a result.

The bottom half on the other hand, used code blocks surrounded by
"```py" formatting brackets which hints to many markdown renders that
syntax highlighting should be applied.

This unifies the code blocks in the top half of the page to use the same
formatting syntax used in the bottom half with syntax highlighting.

This change was prompted by requests to add the syntax highlighting in
comments on the https://devhints.io/python page.

python.md

@@ -5,10 +5,13 @@ category: Python
 
 ### Tuples (immutable)
 
+```py
 tuple = ()
+```
 
 ### Lists (mutable)
 
+```py
 list = []
 list[i:j]  # returns list subset
 list[-1]   # returns last element
@@ -35,10 +38,13 @@ category: Python
 zip(list1, list2)  # returns list of tuples with n-th element of both list1 and list2
 list.sort()        # sorts in-place, returns None
 sorted(list)       # returns sorted copy of list
-    ",".join(list)     # returns a string with list elements seperated by comma
+",".join(list)     # returns a string with list elements separated by comma
+
+```
 
 ### Dict
 
+```py
 dict = {}
 dict.keys()
 dict.values()
@@ -47,18 +53,22 @@ category: Python
 dict.get("key")  # ...this returns None
 dict.setdefault("key", 1)
 **dict           # expands all k/v pairs in place
+```
 
 ### Iteration
 
+```py
 for item in ["a", "b", "c"]:
 for i in range(4):        # 0 to 3
 for i in range(4, 8):     # 4 to 7
 for i in range(1, 9, 2):  # 1, 3, 5, 7
 for key, val in dict.items():
 for index, item in enumerate(list):
+```
 
 ### [String](https://docs.python.org/2/library/stdtypes.html#string-methods)
 
+```py
 str[0:4]
 len(str)
 
@@ -92,25 +102,31 @@ category: Python
     '"Yes," they said.'
 >>> '"Isn\'t," they said.'
     '"Isn\'t," they said.'
+```
 
 ### Casting
 
+```py
 int(str)
 float(str)
 str(int)
 str(float)
 'string'.encode()
+```
 
 ### Comprehensions
 
+```py
 [fn(i) for i in list]            # .map
 map(fn, list)                    # .map, returns iterator
 
 filter(fn, list)                 # .filter, returns iterator
 [fn(i) for i in list if i > 0]   # .filter.map
+```
 
 ### Regex
 
+```py
 import re
 
 re.match(r'^[aeiou]', str)
@@ -120,6 +136,7 @@ category: Python
 expr = re.compile(r'^...$')
 expr.match(...)
 expr.sub(...)
+```
 
 ## File manipulation