Avoid equality check on Watcher

[philippjfr]

Since a Watcher is simply a tuple a check like watcher not in self_._watchers effectively results in an equality comparison on all the objects inside the watcher tuple, including the Parameterized instance the watcher is attached to. Not all Parameterized objects implement equality checks that make sense in this context so to avoid this we only check for identity.

[codecov-commenter]

Codecov Report

Merging #612 (17874f7) into master (becf81b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #612   +/-   ##
=======================================
  Coverage   81.79%   81.79%           
=======================================
  Files           4        4           
  Lines        3027     3027           
=======================================
  Hits         2476     2476           
  Misses        551      551           

Impacted Files	Coverage Δ
param/parameterized.py	81.89% <100.00%> (ø)

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