Change the Magazine number field from an integer to a decimal,
allowing values such as 2005.6

I used the following python code in the django shell to convert
existing numbers like 20056 to the new format:

from decimal import Decimal

for p in PrintIssue.objects.filter(magazine__identifier='yang'):
    s = str(p.number)
    p.number = Decimal("%s.%s" % (s[:-1], s[-1]))
    p.save()