Variable: Fix Variable.copy for StringVariable and TimeVariable

Due to static constructors in Variable.copy and ContinuousVariable.copy
the StringVariable.copy and TimeVariable.copy used to return Variable
and ContinuousVariable instances respectively.

Fix this by dispatching on `type(self)`.

Orange/data/variable.py

@@ -3,7 +3,6 @@
 from math import isnan, floor, sqrt
 import numpy as np
 from pickle import PickleError
-import copy
 
 import collections
 from datetime import datetime, timedelta, timezone
@@ -421,7 +420,7 @@ def __reduce__(self):
         return make_variable, (self.__class__, self._compute_value, self.name), self.__dict__
 
     def copy(self, compute_value):
-        var = Variable(self.name, compute_value)
+        var = type(self)(self.name, compute_value=compute_value)
         var.attributes = dict(self.attributes)
         return var
 
@@ -518,7 +517,7 @@ def repr_val(self, val):
     str_val = repr_val
 
     def copy(self, compute_value=None):
-        var = ContinuousVariable(self.name, self.number_of_decimals, compute_value)
+        var = type(self)(self.name, self.number_of_decimals, compute_value)
         var.attributes = dict(self.attributes)
         return var
 
@@ -899,6 +898,12 @@ def __init__(self, *args, **kwargs):
         self.have_date = 0
         self.have_time = 0
 
+    def copy(self, compute_value=None):
+        copy = super().copy(compute_value=compute_value)
+        copy.have_data = self.have_date
+        copy.have_time = self.have_time
+        return copy
+
     @staticmethod
     def _tzre_sub(s, _subtz=re.compile(r'([+-])(\d\d):(\d\d)$').sub):
         # Replace +ZZ:ZZ with ISO-compatible +ZZZZ, or strip +0000

Orange/tests/test_variable.py

@@ -58,6 +58,7 @@ def test_copy_copies_attributes(self):
         var.attributes["a"] = "b"
         var2 = var.copy(compute_value=None)
         self.assertIn("a", var2.attributes)
+        self.assertIsInstance(var2, type(var))
 
         var2.attributes["a"] = "c"
         # Attributes of original value should not change