> But my question is, then: why is my machine failing this test [the 
> only one which uses this two-part locale] and not the buildbots or 
> (presumably) any other Windows developer?

test_getsetlocale_issue1813 fails for me as well. I can't imagine how setlocale(LC_CTYPE, "tr_TR.ISO8859-9") would succeed with recent versions of the Universal CRT in Windows. It parses "tr_TR" as a BCP 47 locale name, which only supports UTF-8 (e.g. "tr_TR.utf-8") and implicit ANSI (e.g. "tr_TR"). Plus "ISO8859-9" in general isn't a supported encoding of the form ".<codepage>", ".ACP" (ANSI), ".utf8", or ".utf-8". 

With the old CRT (2.x and <=3.4) and older versions of the Universal CRT, the initial locale.setlocale(locale.LC_CTYPE 'tr_TR') call fails as an unsupported locale, so the test is skipped: 

    test_getsetlocale_issue1813 (__main__.TestMiscellaneous) ... skipped 'test needs Turkish locale'

The old CRT only supports "trk_TUR", "trk_Turkey", "turkish_TUR", and "turkish_Turkey".