bpo-38644: Add _PySys_Audit() which takes tstate (GH-19180)

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.