CHG: use deepcopy of StructDict in Welch test

syncopy/tests/test_welch.py

@@ -188,9 +188,9 @@ def test_welch_size_effect(self):
         # Check the number of windows that Welch will average over.
         # To do this, we run mtmconvol and check the output size.
         # This is to verify that the number of windows is equal, and as expected.
-        cfg_mtm_long = cfg_long_no_overlap.copy()
+        cfg_mtm_long = cfg_long_no_overlap.deepcopy()
         cfg_mtm_long.method = "mtmconvol"
-        cfg_mtm_short = cfg_short_with_overlap.copy()
+        cfg_mtm_short = cfg_short_with_overlap.deepcopy()
         cfg_mtm_short.method = "mtmconvol"
 
         spec_long_no_overlap = spy.freqanalysis(cfg_long_no_overlap, wn_long)