weekday() is not consistent with constants

[btschr]

• I am on the latest Pendulum version.
• I have searched the issues of this repo and believe that this is not a duplicate.

• OS version and name: Tested on Ubuntu 20.04 and Windows 10 with same results
• Pendulum version: 2.1.2

Issue

The DateTime.weekday() function does not return values consistent with the weekday constants. Illustration below:

import pendulum
# create a test date (this date falls on a Sunday)
test_date = pendulum.DateTime(2021, 11, 7)

# confirm that this is indeed a Sunday:
print(f'{test_date:%A}')  
# output >> Sunday

# check if pendulum thinks its a Sunday (this FAILS)
print(test_date.weekday() == pendulum.SUNDAY) 
# output >> False

# for some reason, it thinks it's a Saturday
print(test_date.weekday() == pendulum.SATURDAY) 
# output >> True

I believe this is because .weekday() is inherited from datetime.datetime, which means the solution should either be:

1. Create a pendulum.DateTime.weekday() override function that simply returns the .day_of_week property
2. Add another set of constants like DT_MONDAY which corresponds with the datetime.datetime weekdays