datepicker fails to find firstweekday in calendar

[Rich Shepard]

I'm running Python3-3.8.2 on Slackware-14.2/x86_64 and I tried to run Miguel
Martinez Lopez's datepicker.py (version 1.0.7) before learning how I can use
it in my application. It fails to find the attribute in the imported
calendar module:

$ python3 datepicker.py 
Traceback (most recent call last):
   File "datepicker.py", line 53, in <module>
     class Calendar(ttk.Frame):
   File "datepicker.py", line 57, in Calendar
     def __init__(self, master=None, year=None, month=None,
firstweekday=calendar.MONDAY, locale=None, activebackground='#b1dcfb',
activeforeground='black', selectbackground='#003eff',
selectforeground='white', command=None, borderwidth=1, relief="solid",
on_click_month_button=None):
AttributeError: module 'calendar' has no attribute 'MONDAY'

(N.B. The above itiation is line wrapped here but not in the trace back.)

Line 53 starts the calendar class:

class Calendar(ttk.Frame):
     datetime = calendar.datetime.datetime
     timedelta = calendar.datetime.timedelta

     def __init__(self, master=None, year=None, month=None,
firstweekday=calendar.MONDAY, locale=None, activebackground='#b1dcfb',
activeforeground='black', selectbackground='#003eff',
selectforeground='white', command=None, borderwidth=1, relief="solid",
on_click_month_button=None):

(N.B. Local line wrapping; in the code it's all on one line.)

Since calendar is built in to Python3-3.8.2 where do I look for the source
of the problem so I can fix it?

Rich