I would also like to note the following curiosity:

>>> calendar.isleap("%d")
False

I still don't think there is anything worth fixing here, but we can consider replacing year % 4 == 0 predicate with year & 3 == 0 which will change the error raised by isleap("2004") from

TypeError: not all arguments converted during string formatting

to 

TypeError: unsupported operand type(s) for &: 'str' and 'int'

I am not sure if the later is much clearer than the former once you consider obfuscated year & 3 == 0 showing up in the backtrace compared to a clear year % 4 == 0 test.