unpin nixio version as fix was released in version 1.5.3

sprenger · sprenger · commit c0a84aec7cab · 2022-03-25T15:24:49.000+01:00
diff --git a/requirements_testing.txt b/requirements_testing.txt
@@ -6,7 +6,7 @@ h5py
 igor
 klusta
 tqdm
-nixio==1.5.1
+nixio
 matplotlib
 ipython
 coverage
