Support comparing date-like objects

[maximlt]

Discovered this bug while working on holoviz/lumen#276

The Comparator class didn't support datetime.date, datetime.datetime and np.datetime64 object. This meant that, if a parameter value was set to its old value, Param would still see it as a new value and would let potential watchers be executed, while they should not. This caused problems for Panel bidirectional links which don't check for equality but rely on Param.

I see that other types aren't supported, like Numpy Arrays, Pandas DataFrame, or frozensets. If more types should be added, let's come up with a list and add them in another PR.

[codecov-commenter]

Codecov Report

Merging #629 (46d6eb9) into master (d960230) will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #629      +/-   ##
==========================================
+ Coverage   82.91%   83.07%   +0.15%     
==========================================
  Files           5        5              
  Lines        3073     3084      +11     
==========================================
+ Hits         2548     2562      +14     
+ Misses        525      522       -3     

Impacted Files	Coverage Δ
param/parameterized.py	83.84% <100.00%> (+0.51%)	⬆️
numbergen/__init__.py	80.37% <0.00%> (-0.34%)	⬇️

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[maximlt]

@philippjfr I've seen you added the Comparator class, in 6fe66a8 I added an identity check to short cut the equality comparisons. This I think would have fixed the issue I was having with bidirectional links in Panel, since the same attribute object is shared between the linked Parameterized instances.

However this test is failing because the code did not handle any other type than the ones declared in the equalities dictionary.

  _____________ TestWatch.test_triggered_when_unchanged_complex_type _____________
  
  self = <tests.API1.testwatch.TestWatch testMethod=test_triggered_when_unchanged_complex_type>
  
      def test_triggered_when_unchanged_complex_type(self):
          def accumulator(change):
              self.accumulator += 1
      
          obj = SimpleWatchExample()
          obj.param.watch(accumulator, 'a')
          subobj = object()
          obj.a = subobj
          self.assertEqual(self.accumulator, 1)
          obj.a = subobj
  >       self.assertEqual(self.accumulator, 2)
  E       AssertionError: 1 != 2

[philippjfr]

I've removed the identity check, not because I think we shouldn't do it but because it's a change in behavior and we need to consider it carefully. Please open a new issue or PR to discuss further.

[philippjfr]

MacOS tests passed but coveralls errored for some reason. Merging.

[maximlt]

This was a codecov issue (codecov/codecov-action#745) that is now resolved.