crypto._cert_fingerprint: pass bytes to load_pem_x509_certificate

crypto._cert_fingerprint must ensure first parameter
to load_pem_x509_certificate is bytes, as there are
code paths which pass this as a string, such as
crypto.from_keyspec -> crypto._load_keyspec ->
crypto.XMLSecCryptoFromXML.__init__ ->
crypto._cert_fingerprint

src/xmlsec/crypto.py

@@ -358,6 +358,8 @@ def _get_cert_by_fp(self, fp):
 
 def _cert_fingerprint(cert_pem):
     if "-----BEGIN CERTIFICATE" in cert_pem:
+        if isinstance(cert_pem, six.text_type):
+            cert_pem = cert_pem.encode()
         cert = load_pem_x509_certificate(cert_pem, backend=default_backend())
     else:
         cert = load_der_x509_certificate(base64.standard_b64decode(cert_pem), backend=default_backend())