sagemathgh-39405: Resolve error in transversal_matroid.pyx

An error appears on some ``Python`` implementations because of a bug in
``transversal_extension``.
The solution is to ensure that the label of `newset` is not `element`.

Reported to me in
sagemath#39155 (comment).

URL: sagemath#39405
Reported by: gmou3
Reviewer(s): Frédéric Chapoton

build/pkgs/configure/checksums.ini

@@ -1,3 +1,3 @@
 tarball=configure-VERSION.tar.gz
-sha1=86711d4cbef2cd4e7bb4afcde36965e5dea908e0
-sha256=9793cf92ebdceb09050a585294de93c242c033745a0012cae1bd301d307a45df
+sha1=ae4e553c8b43ea4fd5bb3a178a975a71b256f989
+sha256=495e94f3d72b725715f0307b25f781d3c908816a88bf9a891fd3e0e050635b03

build/pkgs/configure/package-version.txt

@@ -1 +1 @@
-efc0914cd8d72a9bdfdcca4511b55e290b9b6674
+1206533093cda4ddd0f5cbe067ab8a7384892a07

src/sage/matroids/transversal_matroid.pyx

@@ -668,7 +668,7 @@ cdef class TransversalMatroid(BasisExchangeMatroid):
             labels.append(l)
         return TransversalMatroid(sets, groundset=self.groundset(), set_labels=labels)
 
-    cpdef transversal_extension(self, element=None, newset=False, sets=[]):
+    cpdef transversal_extension(self, element=None, newset=False, sets=None):
         r"""
         Return a :class:`TransversalMatroid` extended by an element.
 
@@ -751,6 +751,8 @@ cdef class TransversalMatroid(BasisExchangeMatroid):
             Transversal matroid of rank 3 on 5 elements, with 3 sets
             sage: Ne = N.transversal_extension(element='f', sets=['s2'])
         """
+        if sets is None:
+            sets = []
         cdef set parsed_sets = set(sets)
         if element is None:
             element = newlabel(self._groundset)
@@ -781,17 +783,17 @@ cdef class TransversalMatroid(BasisExchangeMatroid):
             else:
                 new_sets.append(s)
 
+        groundset = self._groundset.union([element])
         if newset:
             if newset is True:
-                newset = newlabel(self._groundset.union(labels))
+                newset = newlabel(groundset.union(labels))
             new_sets.append([element])
             labels = list(labels)  # Make a shallow copy since we mutate it
             labels.append(newset)
 
-        groundset = self._groundset.union([element])
         return TransversalMatroid(new_sets, groundset, labels)
 
-    def transversal_extensions(self, element=None, sets=[]):
+    def transversal_extensions(self, element=None, sets=None):
         r"""
         Return an iterator of extensions based on the transversal presentation.
 
@@ -829,7 +831,7 @@ cdef class TransversalMatroid(BasisExchangeMatroid):
             raise ValueError("cannot extend by element already in groundset")
 
         labels = self._set_labels_input
-        if not sets:
+        if sets is None:
             sets = labels
         elif not set(sets).issubset(labels):
             raise ValueError("sets do not match presentation")