Handle error when importing older versions of pyOpenSSL

This commit catches an AttributeError which can occur if the
version of pyOpenSSL on the system is older than 23.0.0. With
this change, an older version is treated the same as not having
pyOpenSSL installed at all. Thanks go to Maximilian Knespel for
reporting this issue and testing the fix!

asyncssh/crypto/__init__.py

@@ -57,5 +57,5 @@
 try:
     from .x509 import X509Certificate, X509Name, X509NamePattern
     from .x509 import generate_x509_certificate, import_x509_certificate
-except ImportError: # pragma: no cover
+except (ImportError, AttributeError): # pragma: no cover
     pass