[python-crypto] M2Crypto Verify CRL Signature

[Randall]

How does one verify a CRL signature using M2Crypto?  I would expect it 
should be similar to the X509.verify method, but it does not exist for 
the CRL class.  X509.verify calls m2.X509_verify, but that function will 
not accept a CRL instance.

If, as it appears, M2Crypto doesn't directly support this, how might one 
go about checking the CRL signature manually?  Given the CRL, how does 
one extract the message (use the DER encoded or PEM or plain-text e.g. 
CRL.as_text() version?) to hash and the sha or md5 hash?