Fix broken imports left over from the BBSE/LEIP commit

quapy/classification/__init__.py imported a labelshift module that doesn't exist
anywhere in the repo (and nothing else references it), breaking `import quapy`
entirely; the import is removed.

quapy/method/aggregative.py imported a nonexistent `_liep` module instead of the
actual file, _liep_draft.py, which itself never got a LEIP class added (only
helper functions such as leip()). Guards the import so LEIP degrades to an
"not available" placeholder, like the other optional neural methods, instead of
crashing the whole package. test_leip/test_leip_fixed_tau still fail as a result;
finishing LEIP is left for a follow-up.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

quapy/classification/__init__.py

@@ -1,4 +1,3 @@
 from . import calibration
-from . import labelshift
 from . import methods
 from . import svmperf

quapy/method/aggregative.py

@@ -2218,9 +2218,11 @@ KDEyML = _kdey.KDEyML
 KDEyHD = _kdey.KDEyHD
 KDEyCS = _kdey.KDEyCS
 
-from . import _liep
-
-LEIP = _liep.LEIP
+try:
+    from . import _liep_draft as _liep
+    LEIP = _liep.LEIP
+except AttributeError:
+    LEIP = "LEIP is not available (incomplete implementation in _liep_draft.py)"
 
 
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