DateTimeList: add assignment operator and default constructor

Author: amadeus84

datetime_utils.h

@@ -64,8 +64,12 @@ template<class Time_t>
 class DateTimeList
 {
 public:
+
+    DateTimeList() = default;
+
     DateTimeList(const Time_t* t, size_t nt)
     {
+        matplotlibcpp::detail::_interpreter::get();
         tlist = (PyListObject*)toPyDateTimeList(t, nt);
     }
 
@@ -74,6 +78,12 @@ class DateTimeList
         if(tlist) Py_DECREF((PyObject*)tlist);
     }
 
+    DateTimeList& operator=(const DateTimeList& rhs) {
+        tlist=rhs.tlist;
+        Py_INCREF(tlist);
+        return *this;
+    }
+
     PyListObject* get() const { return tlist; }
     size_t size() const { return tlist ? PyList_Size((PyObject*)tlist) : 0; }
 private: