Fix exceptions for Morphology.__reduce__ (#5792)

Pickle exceptions in the MORPH_RULES format instead of the internal
format after the recent `Morphology.__init__` changes.

spacy/morphology.pyx

@@ -96,7 +96,7 @@ cdef class Morphology:
 
     def __reduce__(self):
         return (Morphology, (self.strings, self.tag_map, self.lemmatizer,
-                self._exc), None, None)
+                self.exc), None, None)
 
     def add(self, features):
         """Insert a morphological analysis in the morphology table, if not

spacy/tests/morphology/test_morph_pickle.py

@@ -0,0 +1,22 @@
+import pytest
+import pickle
+from spacy.morphology import Morphology
+from spacy.strings import StringStore
+from spacy.lemmatizer import Lemmatizer
+from spacy.lookups import Lookups
+
+
+@pytest.fixture
+def morphology():
+    tag_map = {"A": {"POS": "X"}, "B": {"POS": "NOUN"}}
+    exc = {"A": {"a": {"POS": "VERB"}}}
+    lemmatizer = Lemmatizer(Lookups())
+    return Morphology(StringStore(), tag_map, lemmatizer, exc=exc)
+
+
+def test_morphology_pickle_roundtrip(morphology):
+    b = pickle.dumps(morphology)
+    reloaded_morphology = pickle.loads(b)
+
+    assert morphology.tag_map == reloaded_morphology.tag_map
+    assert morphology.exc == reloaded_morphology.exc