Fix propTypes unnecessarily marked as required

Author: wojtekmaj

src/MonthView/Days.jsx

@@ -73,7 +73,7 @@ export default function Days(props) {
 }
 
 Days.propTypes = {
-  calendarType: isCalendarType.isRequired,
+  calendarType: isCalendarType,
   showFixedNumberOfWeeks: PropTypes.bool,
   showNeighboringMonth: PropTypes.bool,
   ...tileGroupProps,

src/MonthView/WeekNumbers.jsx

@@ -61,7 +61,7 @@ export default function WeekNumbers(props) {
 
 WeekNumbers.propTypes = {
   activeStartDate: PropTypes.instanceOf(Date).isRequired,
-  calendarType: isCalendarType.isRequired,
+  calendarType: isCalendarType,
   onClickWeekNumber: PropTypes.func,
   onMouseLeave: PropTypes.func,
   showFixedNumberOfWeeks: PropTypes.bool,

src/MonthView/Weekdays.jsx

@@ -64,7 +64,7 @@ export default function Weekdays(props) {
 }
 
 Weekdays.propTypes = {
-  calendarType: isCalendarType.isRequired,
+  calendarType: isCalendarType,
   formatShortWeekday: PropTypes.func,
   formatWeekday: PropTypes.func,
   locale: PropTypes.string,