Added solution code

Solutions/4_4/stock.py

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+# stock.py
+
+class Stock(object):
+    '''
+    An instance of a stock holding consisting of name, shares, and price.
+    '''
+    def __init__(self, name, shares, price):
+        self.name   = name
+        self.shares = shares
+        self.price  = price
+
+    def cost(self):
+        '''
+        Return the cost as shares*price
+        '''
+        return self.shares * self.price
+
+    def sell(self, nshares):
+        '''
+        Sell a number of shares
+        '''
+        self.shares -= nshares
+