Do not support the IPO stock for shanghai.

[dfenglei]

I have tested it with below code change.
I have not right to check in.
Please check in the below diff file in order to support the SH IPO stock number.

diff --git a/easytrader/helpers.py b/easytrader/helpers.py
index 577c989..4639f20 100644
--- a/easytrader/helpers.py
+++ b/easytrader/helpers.py
@@ -61,7 +61,7 @@ def get_stock_type(stock_code):
assert type(stock_code) is str, 'stock code need str type'
if stock_code.startswith(('sh', 'sz')):
return stock_code[:2]

• if stock_code.startswith(('50', '51', '60', '90', '110', '113', '132', '204')):
• if stock_code.startswith(('50', '51', '60', '73', '90', '110', '113', '132', '204')):