bpo-32417: Make timedelta arithmetic respect subclasses (#10902)

* Make timedelta return subclass types

Previously timedelta would always return the `date` and `datetime`
types, regardless of what it is added to. This makes it return
an object of the type it was added to.

* Add tests for timedelta arithmetic on subclasses

* Make pure python timedelta return subclass types

* Add test for fromtimestamp with tz argument

* Add tests for subclass behavior in now

* Add news entry.

Fixes:
bpo-32417
bpo-35364

* More descriptive variable names in tests

Addresses Victor's comments

Modules/_datetimemodule.c

@@ -3004,7 +3004,8 @@ add_date_timedelta(PyDateTime_Date *date, PyDateTime_Delta *delta, int negate)
     int day = GET_DAY(date) + (negate ? -deltadays : deltadays);
 
     if (normalize_date(&year, &month, &day) >= 0)
-        result = new_date(year, month, day);
+        result = new_date_subclass_ex(year, month, day,
+                                      (PyObject* )Py_TYPE(date));
     return result;
 }
 
@@ -5166,9 +5167,10 @@ add_datetime_timedelta(PyDateTime_DateTime *date, PyDateTime_Delta *delta,
         return NULL;
     }
 
-    return new_datetime(year, month, day,
-                        hour, minute, second, microsecond,
-                        HASTZINFO(date) ? date->tzinfo : Py_None, 0);
+    return new_datetime_subclass_ex(year, month, day,
+                                    hour, minute, second, microsecond,
+                                    HASTZINFO(date) ? date->tzinfo : Py_None,
+                                    (PyObject *)Py_TYPE(date));
 }
 
 static PyObject *