Signing S/MIME content from Python

Question

I've tried various approaches to signing an email message using Python. There's the M2Crypto library mentioned here but that library no longer builds / works properly with Python 3.4+. There's a newer library cryptography which works well for encrypting - see example here. I have some code that does the signing, but it's ugly:

def signEmailFrom(msg, fromAddr):
    """ Signs the provided email message object with the from address cert (.crt) & private key (.pem) from current dir.
    Returns signed mail string, in format
        Content-Type: application/x-pkcs7-mime; smime-type=signed-data; name="smime.p7m"
        Content-Disposition: attachment; filename="smime.p7m"
        Content-Transfer-Encoding: base64
    See RFC5751 section 3.4
    The reason for choosing this format, rather than multipart/signed, is that it prevents the delivery service
    from applying open/link tracking or other manipulations on the body.
    """
    eml = msg.as_bytes()
    with NamedTemporaryFile('wb') as tmpFile:
        tmpFile.file.write(eml)
        tmpFile.file.close()
        pubCertFile = fromAddr + '.crt'
        privkeyFile = fromAddr + '.pem'
        if os.path.isfile(pubCertFile) and os.path.isfile(privkeyFile):
            myout = subprocess.run(['openssl', 'smime', '-in', tmpFile.name, '-sign', '-signer', pubCertFile, '-inkey', privkeyFile, '-md','sha256', '-nodetach'], capture_output=True)
            if myout.returncode == 0:
                sout = myout.stdout.decode('utf8')
                return sout
            else:
                return None
        else:
            print('Unable to open public and private key files for fromAddr', fromAddr)
            return None

My question:

• Is there way to use pyca/cryptography, or other 3.x compatible library, to produce the signed mail?