Use osmocom.construct.{Bytes,GreedyBytes} for hexstring input support

The upstream construct.{Bytes,GreedyBytes} only support bytes/bytearray
input data for the encoder, while the [newly-created]
osmocom.construct.{Bytes,GreedyBytes} support alternatively hex-string input.

This is important in the context of encoding construct-based types from
JSON, where our osmocom.utils.JsonEncoder will automatically convert any
bytes to hex-string, while re-encoding those hex-strings will fail prior
to this patch.

Change-Id: I1c8df6350c68aa408ec96ff6cd1e405ceb1a4fbb
Closes: OS#6774

pySim/iso7816_4.py

@@ -17,7 +17,7 @@ You should have received a copy of the GNU General Public License
 along with this program.  If not, see <http://www.gnu.org/licenses/>.
 """
 
-from construct import GreedyBytes, GreedyString
+from construct import GreedyString
 from osmocom.tlv import *
 from osmocom.construct import *