work around a change in python 3.13

* python/spot/__init__.py: Unindent the docstring for
formula.__format__.  Because Python 3.13 strips the indentation but
previous version didn't, the following test case failed with Python
3.13.
* tests/python/formulas.ipynb: Adjust to unindented docstring.

python/spot/__init__.py

@@ -360,6 +360,11 @@ class formula:
             raise ValueError("unknown string format: " + format)
 
     def __format__(self, spec):
+        # Some test case print this docstring, and different
+        # Python version will handled indentation differently.
+        # (Python versions before 3.13 don't strip indentation.)
+        # So we cannot indent this until Python 3.13 is thee
+        # smallest version we support.
         """Format the formula according to `spec`.
 
 Parameters

tests/python/formulas.ipynb

@@ -505,7 +505,7 @@
        "<?xml version=\"1.0\" encoding=\"UTF-8\" standalone=\"no\"?>\n",
        "<!DOCTYPE svg PUBLIC \"-//W3C//DTD SVG 1.1//EN\"\n",
        " \"http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd\">\n",
-       "<!-- Generated by graphviz version 2.43.0 (0)\n",
+       "<!-- Generated by graphviz version 2.42.4 (0)\n",
        " -->\n",
        "<!-- Title: G Pages: 1 -->\n",
        "<svg width=\"343pt\" height=\"583pt\"\n",
@@ -1056,7 +1056,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.11.8"
+   "version": "3.12.7"
   }
  },
  "nbformat": 4,