Day of week constants

[DanCardin]

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

In pendulum version 2.1.2 the day of week constants are defined as:

SUNDAY = 0
MONDAY = 1
TUESDAY = 2
WEDNESDAY = 3
THURSDAY = 4
FRIDAY = 5
SATURDAY = 6

In version 3 they're defined as:

class WeekDay(IntEnum):
    MONDAY = 0
    TUESDAY = 1
    WEDNESDAY = 2
    THURSDAY = 3
    FRIDAY = 4
    SATURDAY = 5
    SUNDAY = 6

MONDAY = WeekDay.MONDAY
TUESDAY = WeekDay.TUESDAY
WEDNESDAY = WeekDay.WEDNESDAY
THURSDAY = WeekDay.THURSDAY
FRIDAY = WeekDay.FRIDAY
SATURDAY = WeekDay.SATURDAY
SUNDAY = WeekDay.SUNDAY

Further, this translates into object-API level differences:
2.1.2

>>> imp.today().weekday()
0
>>> pendulum.today().day_of_week
1

vs 3.0.0

>>> pendulum.today().weekday()
0
>>> pendulum.today().day_of_week
<WeekDay.MONDAY: 0>

---

At the very least, this appears to be an undocumented breaking change, if it's otherwise an intended change in API.

but it would be nice for there to be a baked-in part of the API to obtain the original 0=sunday day-of-week integer mapping. That somewhat relates to #63. Except I'm more just looking to retain the original pendulum integer mapping that appears to have changed.