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

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

Changes:

* Rewrite _PyUnicode_InsertThousandsGrouping(): it now requires
  a _PyUnicodeWriter object for the buffer and a Python str object
  for digits.
* Rename FILL() macro to unicode_fill(), convert it to static inline function,
  add "assert(0 <= start);" and rework its code.

Include/unicodeobject.h

@@ -2135,10 +2135,10 @@ PyAPI_FUNC(PyObject *) _PyUnicode_XStrip(
    see Objects/stringlib/localeutil.h */
 #ifndef Py_LIMITED_API
 PyAPI_FUNC(Py_ssize_t) _PyUnicode_InsertThousandsGrouping(
-    PyObject *unicode,
-    Py_ssize_t index,
+    _PyUnicodeWriter *writer,
     Py_ssize_t n_buffer,
-    void *digits,
+    PyObject *digits,
+    Py_ssize_t d_pos,
     Py_ssize_t n_digits,
     Py_ssize_t min_width,
     const char *grouping,