Fix signing code snippets in documentation

Adopt create and verify signature snippets in documentation to
accept data to be signed as bytes instead of strings, as changed
in secure-systems-lab#162.

README.rst

@@ -173,7 +173,7 @@ cryptographic operations.
 
  >>> from securesystemslib.keys import *
 
- >>> data = 'The quick brown fox jumps over the lazy dog'
+ >>> data = b'The quick brown fox jumps over the lazy dog'
  >>> ed25519_key = generate_ed25519_key()
  >>> signature = create_signature(ed25519_key, data)
  >>> rsa_key = generate_rsa_key(2048)
@@ -189,7 +189,7 @@ Verify ECDSA, Ed25519, and RSA Signatures
 
  # Continuing from the previous sections . . .
 
- >>> data = 'The quick brown fox jumps over the lazy dog'
+ >>> data = b'The quick brown fox jumps over the lazy dog'
  >>> ed25519_key = generate_ed25519_key()
  >>> signature = create_signature(ed25519_key, data)
  >>> verify_signature(ed25519_key, signature, data)