gh-139353: Add Objects/unicode_format.c file

* Move PyUnicode_Format() implementation from unicodeobject.c
  to unicode_format.c.
* Replace unicode_modifiable() with _PyUnicode_IsModifiable()
* Add empty lines to have two empty lines between functions.

Author: vstinner

Include/internal/pycore_unicodeobject.h

@@ -11,10 +11,14 @@ extern "C" {
 #include "pycore_fileutils.h"     // _Py_error_handler
 #include "pycore_ucnhash.h"       // _PyUnicode_Name_CAPI
 
+
 // Maximum code point of Unicode 6.0: 0x10ffff (1,114,111).
 #define _Py_MAX_UNICODE 0x10ffff
 
 
+extern int _PyUnicode_IsModifiable(PyObject *unicode);
+
+
 static inline void
 _PyUnicode_Fill(int kind, void *data, Py_UCS4 value,
                 Py_ssize_t start, Py_ssize_t length)
@@ -48,6 +52,28 @@ _PyUnicode_Fill(int kind, void *data, Py_UCS4 value,
     }
 }
 
+static inline int
+_PyUnicode_EnsureUnicode(PyObject *obj)
+{
+    if (!PyUnicode_Check(obj)) {
+        PyErr_Format(PyExc_TypeError,
+                     "must be str, not %T", obj);
+        return -1;
+    }
+    return 0;
+}
+
+static inline int
+_PyUnicodeWriter_WriteCharInline(_PyUnicodeWriter *writer, Py_UCS4 ch)
+{
+    assert(ch <= _Py_MAX_UNICODE);
+    if (_PyUnicodeWriter_Prepare(writer, 1, ch) < 0)
+        return -1;
+    PyUnicode_WRITE(writer->kind, writer->data, writer->pos, ch);
+    writer->pos++;
+    return 0;
+}
+
 
 /* --- Characters Type APIs ----------------------------------------------- */
 

Makefile.pre.in

@@ -557,9 +557,10 @@ OBJECT_OBJS=	\
 		Objects/tupleobject.o \
 		Objects/typeobject.o \
 		Objects/typevarobject.o \
+		Objects/unicode_format.o \
 		Objects/unicode_formatter.o \
-		Objects/unicodeobject.o \
 		Objects/unicodectype.o \
+		Objects/unicodeobject.o \
 		Objects/unionobject.o \
 		Objects/weakrefobject.o \
 		@PERF_TRAMPOLINE_OBJ@
@@ -2105,6 +2106,7 @@ Objects/bytes_methods.o: $(srcdir)/Objects/bytes_methods.c $(BYTESTR_DEPS)
 Objects/bytesobject.o: $(srcdir)/Objects/bytesobject.c $(BYTESTR_DEPS)
 Objects/bytearrayobject.o: $(srcdir)/Objects/bytearrayobject.c $(BYTESTR_DEPS)
 
+Objects/unicode_format.o: $(srcdir)/Objects/unicode_format.c $(UNICODE_DEPS)
 Objects/unicodeobject.o: $(srcdir)/Objects/unicodeobject.c $(UNICODE_DEPS)
 
 Objects/dictobject.o: $(srcdir)/Objects/stringlib/eq.h