Fix compatibility with Python 2.7 and improve warning message.
Fix compatibility with Python 2.7 and improve warning message.

Now the rule identifier is printed as part of the warning message, the string identifier is not descriptive enough in many cases.

wxsBSD/yara-python,VirusTotal/yara-python,VirusTotal/yara-python,plusvic/yara-python,wxsBSD/yara-python,plusvic/yara-python