[3.7] bpo-29571: Fix test_re.test_locale_flag() (GH-12178)

Use locale.getpreferredencoding() rather than locale.getlocale() to
get the locale encoding. With some locales, locale.getlocale()
returns the wrong encoding.

For example, on Fedora 29, locale.getlocale() returns ISO-8859-1
encoding for the "en_IN" locale, whereas
locale.getpreferredencoding() reports the correct encoding: UTF-8.

On Windows, set temporarily the LC_CTYPE locale to the user preferred
encoding to ensure that it uses the ANSI code page, to be consistent
with locale.getpreferredencoding().