Remove get_random_bytes from cryptography backend

The RAND_bytes binding has been removed in cryptography 45.0. The
recommendation[1] is now to rely on `os.urandom`, which is already
implemented in the native backend.
The pycrpto implementation was removed earlier, so this removes the
leftover attempt to import it.

Closes: #380

[1] https://cryptography.io/en/latest/random-numbers/

Author: mweinelt

jose/backends/__init__.py

@@ -1,10 +1,4 @@
-try:
-    from jose.backends.cryptography_backend import get_random_bytes  # noqa: F401
-except ImportError:
-    try:
-        from jose.backends.pycrypto_backend import get_random_bytes  # noqa: F401
-    except ImportError:
-        from jose.backends.native import get_random_bytes  # noqa: F401
+from jose.backends.native import get_random_bytes  # noqa: F401
 
 try:
     from jose.backends.cryptography_backend import CryptographyRSAKey as RSAKey  # noqa: F401

jose/backends/cryptography_backend.py

@@ -26,33 +26,11 @@
     long_to_base64,
 )
 from .base import Key
+from . import get_random_bytes
 
 _binding = None
 
 
-def get_random_bytes(num_bytes):
-    """
-    Get random bytes
-
-    Currently, Cryptography returns OS random bytes. If you want OpenSSL
-    generated random bytes, you'll have to switch the RAND engine after
-    initializing the OpenSSL backend
-    Args:
-        num_bytes (int): Number of random bytes to generate and return
-    Returns:
-        bytes: Random bytes
-    """
-    global _binding
-
-    if _binding is None:
-        _binding = Binding()
-
-    buf = _binding.ffi.new("char[]", num_bytes)
-    _binding.lib.RAND_bytes(buf, num_bytes)
-    rand_bytes = _binding.ffi.buffer(buf, num_bytes)[:]
-    return rand_bytes
-
-
 class CryptographyECKey(Key):
     SHA256 = hashes.SHA256
     SHA384 = hashes.SHA384