pyOpenSSL's PKCS7 object provide very little information, how can I get the sha1 digest of the public key in the signature

I would like to parse android apk's CERT.RSA in Python. I know it can be parsed with pyOpenSSL

import OpenSSL

cert = OpenSSL.crypto.load_certificate(OpenSSL.crypto.FILETYPE_ASN1, open('CERT.RSA', 'rb').read())

cert = OpenSSL.crypto.load_pkcs7_data(type, buffer)

cert is of type 'OpenSSL.crypto.PKCS7'.

BUT right now PKCS7 object is not complete, I cannot get attributes I need, is there any alternative way to parse that file?