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

In versions 42.0.0 and 42.0.1, cryptography.io accepted only bytes for
arguments in various methods like `RSAPrivateKey.sign()`. This bug was
fixed in v42.0.2, but let's fix Gramine to supply bytes instead of
bytearray, so that `Sigstruct.to_bytes()` doesn't fail on those affected
versions of cryptography.io.

Corresponding upstream commit in `pyca/cryptography` repository that
introduced the bug: 0000b402dd5edffa6a86ca560464e83a66fd91f5, and commit
that fixed this bug: 08b24d87a64734ac7f5c575b309ad7d49c246353

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