[3.5] bpo-34656: Avoid relying on signed overflow in _pickle memos (GH-9261)

[vstinner]

(cherry picked from commit a4ae828)

https://bugs.python.org/issue34656

[vstinner]

Oh, FTP tests are failing on Travis CI :-( I should backport this fix:

commit c11b3b19a5b022c6c229043d37f9a9fd06f22500
Author: Victor Stinner <vstinner@redhat.com>
Date:   Wed Dec 5 01:58:31 2018 +0100

    [bpo-35411](https://bugs.python.org/issue35411): Skip test_urllib2net FTP tests on Travis CI (GH-10907)
    
    On Travis CI, FTP tests of test_urllib2net randomly fail with "425
    Security: Bad IP connecting".
    
    test.pythoninfo now also logs TRAVIS environment variable.

[vstinner]

Oh, FTP tests are failing on Travis CI :-( I should backport this fix:

Done: I created PR #11874.

[vstinner]

@larryhastings "ftplib.error_temp: 425 Security: Bad IP connecting." bites again, you need a new merge to get my FTP fix.

[bedevere-bot]

@larryhastings: Please replace # with GH- in the commit message next time. Thanks!

[larryhastings]

Thanks for the backported fix! It does my heart good to see pickle avoiding integer overflows.