remove walrus

So far, frappy is still supposed to be 3.6 compatible

Change-Id: Ieb56c6500d58cdc1fe2f34b1448b05ba74b0fa6e
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/34326
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>
Reviewed-by: Georg Brandl <g.brandl@fz-juelich.de>

frappy/lib/__init__.py

@@ -107,7 +107,8 @@ class GeneralConfig:
             if cfg.get('confdir') is None:
                 cfg['confdir'] = path.dirname(configfile)
         for key in mandatory:
-            if (env := environ.get(f'FRAPPY_{key.upper()}')) is not None:
+            env = environ.get(f'FRAPPY_{key.upper()}')
+            if env is not None:
                 cfg[key] = env
         missing_keys = [
             key for key in mandatory