bpo-36722: Don't define ALT_SOABI for Py_TRACE_REFS build

[vstinner]

Py_TRACE_REFS ABI is incompatible with release and debug (Py_DEBUG)
ABI.

https://bugs.python.org/issue36722

[vstinner]

I checked manually pyconfig.h in these 3 configurations. It now works as expected. Moreover, pyport.h ensures that ALT_SOABI is not defined when Py_TRACE_REFS is defined, just in case.

./configure:

/* #undef ALT_SOABI */
/* #undef Py_DEBUG */
/* #undef Py_TRACE_REFS */

./configure --with-pydebug:

#define ALT_SOABI "cpython-38-x86_64-linux-gnu"
#define Py_DEBUG 1
/* #undef Py_TRACE_REFS */

./configure --with-trace-refs:

/* #undef ALT_SOABI */
/* #undef Py_DEBUG */
#define Py_TRACE_REFS 1

ALT_SOABI is only defined when Py_DEBUG is defined, but not when Py_TRACE_REFS is defined: as expected.