fix and check shifting issue
The exception raised by << and >> when shifting mark_t by too many bits are only enabled in SPOT_DEBUG, as those operations are quite low-level. However we were always testing them, and although we wanted them to be active in Python, it was not always the case. * spot/twa/acc.hh: introduce max_accsets() as a static constexpr method, so we can see it in Python. * spot/misc/bitset.hh: Fix preprocessing directive so the check is actually enabled when compiling the Python bindings. * bin/autcross.cc, bin/autfilt.cc, bin/ltlcross.cc: Use max_accsets(). * tests/core/acc.cc: Comment out the shifting exception when SPOT_DEBUG is unset. * tests/python/except.py: Make sure the exception is always raised in Python.