Enable Python RSA backend as a fallback.

Author: Gasper Zejn

jose/backends/__init__.py

@@ -2,7 +2,10 @@
 try:
     from jose.backends.pycrypto_backend import RSAKey
 except ImportError:
-    from jose.backends.cryptography_backend import CryptographyRSAKey as RSAKey
+    try:
+        from jose.backends.cryptography_backend import CryptographyRSAKey as RSAKey
+    except ImportError:
+        from jose.backends.rsa_backend import RSAKey
 
 try:
     from jose.backends.cryptography_backend import CryptographyECKey as ECKey