sccfilter: some inherently-weak automata should have t acceptance

* spot/twaalgos/sccfilter.cc: If an inherently-weak automaton has
no rejecting cycle, reduce its acceptance to t instead of Büchi.
* spot/twa/acc.hh (operator==, operator<): Fix comparisons of
true acceptances.
* NEWS: Mention these two changes.
* spot/twaalgos/sccfilter.hh: Update documentation.
* spot/twaalgos/determinize.cc (tgba_determinize): The call
to scc_filter assume that the input BA is never reduced to t
acceptance.  Call scc_filter with an extra option to ensure that.
* spot/twaalgos/postproc.cc (do_scc_filter): Adjust to add the
extra option when we want to build Büchi or coBuchi.
(ensure_ba): Don't mark trivial SCCs as accepting.
* tests/core/complement.test, tests/core/dstar.test,
tests/core/ltlsynt.test, tests/core/readsave.test,
tests/core/wdba2.test, tests/python/_product_susp.ipynb,
tests/python/automata-io.ipynb, tests/python/dualize.py,
tests/python/highlighting.ipynb, tests/python/intrun.py,
tests/python/setacc.py, tests/python/simstate.py,
tests/python/stutter-inv.ipynb, tests/python/zlktree.py: Adjust test
cases.

NEWS

@@ -108,6 +108,15 @@ New in spot 2.11.6.dev  (not yet released)
     removal of superfluous APs that is now performed by ltlsynt
     (search for --polarity and --global-equivalence above).
 
+  - scc_filter used to reduce automata tagged with the inherently-weak
+    property to weak Büchi automata (unless the acceptance was already
+    t or co-Büchi).  In case where the input automaton had no
+    rejecting cycle, the Büchi acceptance was overkill: scc_filter
+    will now use "t" acceptance.  This change may have unexpected
+    conseqences in code paths that assume running scc_filter on a
+    Büchi automaton will always return a Büchi automaton.  For those,
+    a "keep_one_color" option has been added to scc_filter.
+
   - ltsmin's interface will now point to README.ltsmin in case an
     error is found while running divine or spins.
 
@@ -143,6 +152,10 @@ New in spot 2.11.6.dev  (not yet released)
   - The automaton parser forgot to update the list of highlighted
     edges while dropping edges labeled by bddfalse.  (issue #548.)
 
+  - The comparison operators for acceptance condition (==, !=)
+    could fail to equate two "t" condition, because we have two ways
+    to represent "t": the empty condition, or the empty "Inf({})".
+
 New in spot 2.11.6  (2023-08-01)
 
   Bug fixes: