Fix TelescopeParameter for traitlets 5.1

[maxnoe]

This removal in traitlets seems to be the culprit:

@@ -2644,12 +2638,6 @@ class List(Container):
 
         return super(List, self).validate_elements(obj, value)
 
-    def set(self, obj, value):
-        if isinstance(value, str):
-            return super().set(obj, [value])
-        else:
-            return super().set(obj, value)
-
 

[codecov]

Codecov Report

Merging #1784 (4a0a483) into master (a4b8e54) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1784   +/-   ##
=======================================
  Coverage   91.84%   91.84%           
=======================================
  Files         187      187           
  Lines       14827    14829    +2     
=======================================
+ Hits        13618    13620    +2     
  Misses       1209     1209           

Impacted Files	Coverage Δ
ctapipe/core/traits.py	95.12% <100.00%> (+0.03%)	⬆️

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