Accept lowercase IBANs in iban()

[sayantanmandal1]

iban() treats lowercase input as well-formed (the regex has re.IGNORECASE and
[a-z0-9]) but then rejects it:

iban("GB82WEST12345698765432")  # True
iban("gb82west12345698765432")  # False, same IBAN

_char_value scores letters with 10 + ord(char) - ord("A"), which is only right
for uppercase, so lowercase letters get the wrong value and the mod-97 check
fails.

Upper-casing the char before scoring it fixes this. It only lets through inputs
that were already valid IBANs, so nothing that passed before changes. Added
lowercase and mixed-case cases to the tests.