[python] Pass bytes to cryptography.io's RSAPrivateKey.sign()

Until RIIR, cryptography.io accepted bytearray for arguments in various
methods like `RSAPrivateKey.sign()`. Technically at least `.sign()` is
documented to accept bytes only (even if bytearray is "bytes-like").
Fix by always converting to bytes in `Sigstruct.to_bytes()` (which BTW
should return bytes, as the name suggests).

Corresponding upstream commit in `pyca/cryptography` repository:
0000b402dd5edffa6a86ca560464e83a66fd91f5

Signed-off-by: Wojtek Porczyk <woju@invisiblethingslab.com>

python/graminelibos/sigstruct.py

@@ -116,7 +116,7 @@ def to_bytes(self, verify=True, verify_sig_fields=False):
                 struct.pack_into(fmt, buffer, offset, int(f'{self[key]}', 16))
                 continue
             struct.pack_into(fmt, buffer, offset, self[key])
-        return buffer
+        return bytes(buffer)
 
 
     @classmethod