[issue36346] Prepare for removing the legacy Unicode C API

[Inada Naoki]

Inada Naoki <songofacandy at gmail.com> added the comment:

Oh, why I can not use C99?

```
/home/buildbot/buildarea/3.x.cstratak-RHEL7-ppc64le/build/Include/cpython/unicodeobject.h: In function ‘Py_UNICODE_FILL’:
/home/buildbot/buildarea/3.x.cstratak-RHEL7-ppc64le/build/Include/cpython/unicodeobject.h:56:5: error: ‘for’ loop initial declarations are only allowed in C99 mode
     for (Py_ssize_t i = 0; i < length; i++) {
     ^
/home/buildbot/buildarea/3.x.cstratak-RHEL7-ppc64le/build/Include/cpython/unicodeobject.h:56:5: note: use option -std=c99 or -std=gnu99 to compile your code
```

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<https://bugs.python.org/issue36346>
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