tools: make print_table compatible with python2 and python3 tests

StringIO expect 'str' object, but on python2 it's different from
'unicode' (returned by decode() method). Convert it explicitly back to
str.

qubesmgmt/tools/__init__.py

@@ -474,7 +474,7 @@ def print_table(table, stream=None):
             stdout=subprocess.PIPE)
         p.stdin.write(text_table.encode())
         (out, _) = p.communicate()
-        stream.write(out.decode())
+        stream.write(str(out.decode()))
     else:
         p = subprocess.Popen(cmd, stdin=subprocess.PIPE)
         p.communicate(text_table.encode())