Raise AttributeError when attempting to set value on an rx obj

[maximlt]

Resolves https://github.com/holoviz/param/issues/869

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 88.13%. Comparing base (b95d476) to head (ae47262). :warning: Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1022   +/-   ##
=======================================
  Coverage   88.12%   88.13%           
=======================================
  Files           9        9           
  Lines        4857     4861    +4     
=======================================
+ Hits         4280     4284    +4     
  Misses        577      577           

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