bpo-33954: Fix _PyUnicode_InsertThousandsGrouping() (GH-10623) (GH-10718)

bpo-33954: Fix _PyUnicode_InsertThousandsGrouping() (GH-10623) (GH-10718)

Fix str.format(), float.__format__() and complex.__format__() methods
for non-ASCII decimal point when using the "n" formatter.

Rewrite _PyUnicode_InsertThousandsGrouping(): it now requires
a _PyUnicodeWriter object for the buffer and a Python str object
for digits.

(cherry picked from commit 59423e3ddd736387cef8f7632c71954c1859bed0)