bpo-41428: Fix compiler warning in unionobject.c (GH-22416)

Use Py_ssize_t type rather than int, to store lengths in
unionobject.c. Fix the warning:

Objects\unionobject.c(205,1): warning C4244: 'initializing':
conversion from 'Py_ssize_t' to 'int', possible loss of data
1 file changed