I have not found a way to load an RSA private key from a PEM file to use it in python-crypto (signature).
python-openssl can load a PEM file but the PKey object can’t be used to retrieved key information (p, q, …) to use with Crypto.PublicKey.construct().
is this (close to) what you tried doing?
That should work. The issue might be with openssl too, does it work when you just use openssl (not in Python)?
Link to Me Too Crypto