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

@@ -1,61 +1,63 @@
 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 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):
+    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):
+    def insert_head(Head, data):
         tamp = Head
-        if (tamp == None):
-            newNod = Node()#create a new Node
+        if tamp is None:
+            newNod = Node()  # create a new Node
             newNod.data = data
             newNod.next = None
-            Head = newNod#make new node to Head       
+            Head = newNod  # make new node to Head
         else:
             newNod = Node()
             newNod.data = data
-            newNod.next = Head#put the Head at NewNode Next
-            Head=newNod#make a NewNode to Head
-        return Head   
-    
-    def printList(Head):#print every node data
-        tamp=Head
-        while tamp!=None:
+            newNod.next = Head  # put the Head at NewNode Next
+            Head = newNod  # make a NewNode to Head
+        return Head
+
+    def printList(Head):  # print every node data
+        tamp = Head
+        while tamp is not None:
             print(tamp.data)
-            tamp=tamp.next
-    
-    def delete_head(Head):#delete from head
-        if Head!=None:
-            Head=Head.next
-        return Head#return new Head
-    
-    def delete_tail(Head):#delete from tail
-        if Head!=None:
+            tamp = tamp.next
+
+    def delete_head(Head):  # delete from head
+        if Head is not None:
+            Head = Head.next
+        return Head  # return new Head
+
+    def delete_tail(Head):  # delete from tail
+        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):
-        return Head is None #Return if Head is none
-    
+        return Head is None  # Return if Head is none
+
     def reverse(Head):
         prev = None
         current = Head
-        
+
         while(current):
-            # Store the current node's next node. 
+            # Store the current node's next node.
             next_node = current.next
             # Make the current node's next point backwards
             current.next = prev
@@ -63,5 +65,5 @@ class Linked_List:
             prev = current
             # Make the current node the next node (to progress iteration)
             current = next_node
-        # Return prev in order to put the head at the end 
+        # Return prev in order to put the head at the end
         Head = prev