fix validate_digits actually allowing non digit characters

[romaincaillon]

str.isdigit method actually allows non digit characters like ² or ⑥.

>>> from pydantic_extra_types.payment import PaymentCardNumber
>>> PaymentCardNumber("²")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/rcaillon/.local/lib/python3.10/site-packages/pydantic_extra_types/payment.py", line 47, in __init__
    card_number = self.validate_luhn_check_digit(card_number)
  File "/home/rcaillon/.local/lib/python3.10/site-packages/pydantic_extra_types/payment.py", line 108, in validate_luhn_check_digit
    sum_ = int(card_number[-1])
ValueError: invalid literal for int() with base 10: '²'
>>> PaymentCardNumber("⑥")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/rcaillon/.local/lib/python3.10/site-packages/pydantic_extra_types/payment.py", line 47, in __init__
    card_number = self.validate_luhn_check_digit(card_number)
  File "/home/rcaillon/.local/lib/python3.10/site-packages/pydantic_extra_types/payment.py", line 108, in validate_luhn_check_digit
    sum_ = int(card_number[-1])
ValueError: invalid literal for int() with base 10: '⑥'

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (084ce94) 100.00% compared to head (0e8efea) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #120   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           10        10           
  Lines          688       688           
  Branches       176       176           
=========================================
  Hits           688       688           

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[yezz123]

LGTM! Thanks 🙏🏻