What am I doing wrong?

[Larz60+]

def find_leap(year):
    if (year % 100 != 0 and year % 4 == 0) or
        (year % 100 == 0 and year % 400 == 0):
        return True
    return False

Not tested.
I would create the test file with an editor as it is not required, at least in your spec above

To read the file, use:

with open('all_years.txt') as f:
    year = f.readlines()
    if find_leap(year.strip()):
        # ... add print here