GUI fixes

- do not show command result dialog when result is None
- apply fmtstr, if available

+ fix io import in secop.core
+ change old style <basecls>.__init__(self, ...) calls to super().__init__(...)

Change-Id: I599d5d8e8ff430ea9454a0858d703290e87454fc
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/26397
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Jens Krueger <jens.krueger@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

secop/core.py

@@ -36,4 +36,4 @@ from secop.params import Command, Parameter
 from secop.poller import AUTO, DYNAMIC, REGULAR, SLOW
 from secop.properties import Property
 from secop.proxy import Proxy, SecNode, proxy_class
-from secop.stringio import HasIodev, StringIO
+from secop.io import HasIodev, StringIO, BytesIO