bpo-38644: Add _PySys_Audit() which takes tstate

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Add _PySys_Audit() function to the internal C API: similar to
PySys_Audit(), but requires a mandatory tstate parameter.

Cleanup sys_audit_tstate() code: remove code path for NULL tstate,
since the function exits at entry if tstate is NULL. Remove also code
path for NULL tstate->interp: should_audit() now ensures that it is
not NULL (even if tstate->interp cannot be NULL in practice).

PySys_AddAuditHook() now checks if tstate is not NULL to decide if
tstate can be used or not, and tstate is set to NULL if the runtime
is not initialized yet.

Use _PySys_Audit() in sysmodule.c.

https://bugs.python.org/issue38644

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Adding extern void _PySys_ClearAuditHooks(PyThreadState *tstate); to Include/internal/pycore_sysmodule.h doesn't export the symbol. Python is now built using -fvisibility=hidden compiler flag which is supported by GCC and clang. Symbols which are not exported explicitly are hidden by default. If the compiler doesn't support -fvisibility=hidden, _PySys_ClearAuditHooks() should not be exported neither. At least, I don't expect my change to change anything.

I prefer to add the function definition in a header file to ensure that tstate parameter is passed properly (as a pointer, not as an int).

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cc @zooba