gh-37410: minor fixes in sandpiles

Just fixing a few `ruff` warnings in the two modified files, plus some
`pep8` code formatting.

Also a few code simplifications on the way.

I also changed some errors from `UserWarning` to better ones.

and changed one function, used nowhere inside sage, into an iterator

### 📝 Checklist

- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.

URL: #37410
Reported by: Frédéric Chapoton
Reviewer(s): Matthias Köppe

Author: Release Manager

build/pkgs/configure/checksums.ini

@@ -1,4 +1,4 @@
 tarball=configure-VERSION.tar.gz
-sha1=0c93ffbc40ee55c429b82c6070817c0133a05013
-md5=96e8d0c3aa48cab18b60499c4d303a73
-cksum=2523075193
+sha1=cebcfe9542c5aa28fd27329786ec7ceb53c4b18b
+md5=83b5e5a5ad023e6266626dfde92f5969
+cksum=383438152

build/pkgs/configure/package-version.txt

@@ -1 +1 @@
-81a374dc4dacba71cf071f981d496ff63b2b2acb
+f531179bcd7b18e31291000415f0d076538a0802

src/sage/sandpiles/examples.py

@@ -25,7 +25,8 @@
 from sage.sandpiles.sandpile import Sandpile
 from sage.graphs.graph_generators import graphs
 
-class SandpileExamples():
+
+class SandpileExamples:
     """
     Some examples of sandpiles.
 
@@ -92,7 +93,7 @@ def Complete(self, n):
             sage: sandpiles.Complete(3) == sandpiles.Cycle(3)
             True
         """
-        return Sandpile(graphs.CompleteGraph(n),0)
+        return Sandpile(graphs.CompleteGraph(n), 0)
 
     def Cycle(self, n):
         """
@@ -119,7 +120,7 @@ def Cycle(self, n):
              (3, 0, 1),
              (3, 2, 1)]
         """
-        return Sandpile(graphs.CycleGraph(n),0)
+        return Sandpile(graphs.CycleGraph(n), 0)
 
     def Diamond(self):
         """
@@ -164,18 +165,18 @@ def Fan(self, n, deg_three_verts=False):
         """
         f = graphs.WheelGraph(n)
         if n > 2:
-            f.delete_edge(1,n-1)
+            f.delete_edge(1, n-1)
             if deg_three_verts:
                 f.allow_multiple_edges(True)
-                f.add_edges([(0,1),(0,n-1)])
-            return Sandpile(f,0)
+                f.add_edges([(0, 1), (0, n-1)])
+            return Sandpile(f, 0)
         elif n == 1:
-            return Sandpile(f,0)
+            return Sandpile(f, 0)
         elif n == 2:
             if deg_three_verts:
-                return Sandpile({0:{1:3}, 1:{0:3}})
+                return Sandpile({0: {1: 3}, 1: {0: 3}})
             else:
-                return Sandpile(f,0)
+                return Sandpile(f, 0)
 
     def Grid(self, m, n):
         """
@@ -200,11 +201,12 @@ def Grid(self, m, n):
             sage: s.dict()
             {(0, 0): {(1, 1): 4}, (1, 1): {(0, 0): 4}}
         """
-        G = graphs.Grid2dGraph(m+2,n+2)
+        G = graphs.Grid2dGraph(m+2, n+2)
         G.allow_multiple_edges(True)  # to ensure each vertex ends up with degree 4
-        V = [(i,j) for i in [0,m+1] for j in range(n+2)] + [(i,j) for j in [0,n+1] for i in range(m+2)]
+        V = [(i, j) for i in [0, m+1] for j in range(n+2)]
+        V += [(i, j) for j in [0, n+1] for i in range(m+2)]
         G.merge_vertices(V)
-        return Sandpile(G, (0,0))
+        return Sandpile(G, (0, 0))
 
     def House(self):
         """
@@ -224,7 +226,7 @@ def House(self):
             sage: s.invariant_factors()
             [1, 1, 1, 11]
         """
-        return Sandpile(graphs.HouseGraph(),0)
+        return Sandpile(graphs.HouseGraph(), 0)
 
     def Wheel(self, n):
         """
@@ -244,7 +246,7 @@ def Wheel(self, n):
             sage: w.invariant_factors()
             [1, 1, 1, 11, 11]
         """
-        return Sandpile(graphs.WheelGraph(n),0)
+        return Sandpile(graphs.WheelGraph(n), 0)
 
 
 sandpiles = SandpileExamples()