bpo-46670: Define all macros for stringlib

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bytesobject.c, bytearrayobject.c and unicodeobject.c now define all
macros used by stringlib, to avoid using undefined macros.
Fix "gcc -Wundef" warnings.

https://bugs.python.org/issue46670

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cc @serhiy-storchaka: I already merged my PR, but you may want to have a look ;-)

[serhiy-storchaka]

Was it needed to define STRINGLIB_MUTABLE?

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Was it needed to define STRINGLIB_MUTABLE?

I prefer to always define the macro to reduce the risk of bugs related to typos: #ifndef STRNIGLIB_MUTABLE is false because of a typo in the variable nane, whereas #if STRNIGLIB_MUTABLE == 0 emits a warning when Python is built with -Wundef: you can catch the typo.

stringlib uses many macros, the risk of forgetting to define a macro and the risk of a typo is high.

I adopted a smilar strategy in my #31171 change which replaces #ifndef PY_NO_SHORT_FLOAT_REPR with #if _PY_SHORT_FLOAT_REPR == 0. For this macro, it's worse because the macro is not define if you forget to include pycore_pymath.h!