Fix unresolved name: insert_tail()

insert_tail(Head.next, data) --> Head.next.insert_tail(data)

Fixes: $ __flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics__
```
./data_structures/LinkedList/singly_LinkedList.py:14:13: F821 undefined name 'insert_tail'
            insert_tail(Head.next, data)
            ^
1     F821 undefined name 'insert_tail'
```

Also formats the code to be compliant with [PEP8](http://pep8.org).

data_structures/LinkedList/singly_LinkedList.py

@@ -1,21 +1,22 @@
 from __future__ import print_function
+
+
 class Node:  # create a Node
     def __int__(self, data):
         self.data = data  # given data
         self.next = None  # given next to None
+
+
 class Linked_List:
-    
-    pass
-    
     def insert_tail(Head, data):
         if(Head.next is None):
             Head.next = Node(data)
         else:
-            insert_tail(Head.next, data)
+            Head.next.insert_tail(data)
 
     def insert_head(Head, data):
         tamp = Head
-        if (tamp == None):
+        if tamp is None:
             newNod = Node()  # create a new Node
             newNod.data = data
             newNod.next = None
@@ -29,22 +30,23 @@ class Linked_List:
 
     def printList(Head):  # print every node data
         tamp = Head
-        while tamp!=None:
+        while tamp is not None:
             print(tamp.data)
             tamp = tamp.next
 
     def delete_head(Head):  # delete from head
-        if Head!=None:
+        if Head is not None:
             Head = Head.next
         return Head  # return new Head
 
     def delete_tail(Head):  # delete from tail
-        if Head!=None:
+        if Head is not None:
             tamp = Node()
             tamp = Head
-            while (tamp.next).next!= None:#find the 2nd last element
+            while (tamp.next).next is not None:  # find the 2nd last element
                 tamp = tamp.next
-            tamp.next=None#delete the last element by give next None to 2nd last Element
+            # delete the last element by give next None to 2nd last Element
+            tamp.next = None
         return Head
 
     def isEmpty(Head):