fix phone export

cntmanage/flightslot/actions/exportweek.py

@@ -80,7 +80,7 @@ def export_selected(request: HttpRequest, queryset: QuerySet[WeekPreference]) ->
     row_offset: int = 0
     for i, q in enumerate(queryset.order_by("week", "student__surname", "student__name", "student__course"), start=1):
         student_data: List[str]
-        student_phone: str = q.student.phone if q.student.phone else ""
+        student_phone: str = str(q.student.phone) if q.student.phone else ""
         student_email: str = q.student.email
         student_course_type: str
         student_course_number: str