python 3 兼容性修复，string.ascii_letters (Thanks DelightRun)

Author: doraemonext

wechat_sdk/lib/crypto/base.py

@@ -79,5 +79,5 @@ def get_random_str(self):
 
         @return: 16位字符串
         """
-        rule = string.letters + string.digits
+        rule = string.ascii_letters + string.digits
         return "".join(random.sample(rule, 16))