bpo-41428: Fix compiler warning in unionobject.c

[vstinner]

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

https://bugs.python.org/issue41428

[corona10]

lgtm

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Ubuntu Shared 3.x has failed when building commit d73cf7c.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/all/#builders/506/builds/124) and take a look at the build logs.
4. Check if the failure is related to this commit (d73cf7c) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/all/#builders/506/builds/124

Summary of the results of the build (if available):

Click to see traceback logs

remote: Enumerating objects: 4, done.        
remote: Counting objects:  25% (1/4)        
remote: Counting objects:  50% (2/4)        
remote: Counting objects:  75% (3/4)        
remote: Counting objects: 100% (4/4)        
remote: Counting objects: 100% (4/4), done.        
remote: Total 4 (delta 3), reused 4 (delta 3), pack-reused 0        
From https://github.com/python/cpython
 * branch                  master     -> FETCH_HEAD
Reset branch 'master'

Parser/parser.c:24830:1: fatal error: error writing to /tmp/ccadKSXO.s: No space left on device
 }
 ^
compilation terminated.
make: *** [Parser/parser.o] Error 1

find: ‘build’: No such file or directory
find: ‘build’: No such file or directory
find: ‘build’: No such file or directory
find: ‘build’: No such file or directory
make: [clean-retain-profile] Error 1 (ignored)