bpo-41428: Fix compiler warnings in unionobject.c (GH-22388)

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

Objects\unionobject.c(189,71): warning C4244: '+=':
conversion from 'Py_ssize_t' to 'int', possible loss of data

Objects\unionobject.c(182,1): warning C4244: 'initializing':
conversion from 'Py_ssize_t' to 'int', possible loss of data

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

Objects\unionobject.c(437,1): warning C4244: 'initializing':
conversion from 'Py_ssize_t' to 'int', possible loss of data