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.