Tweak f-string to avoid flake8 E226

❯ flake8 Tests/test_Phylo_igraph.py
Tests/test_Phylo_igraph.py:34:34: E226 missing whitespace around arithmetic operator

Perhaps we should explicitly add E226 to our flake8 ignore?
Ruff didn't mind either way.

Tests/test_Phylo_igraph.py

@@ -31,7 +31,7 @@ def test_to_igraph_simple(self):
         """Tree to Graph conversion, simple tree."""
         tree = Phylo.read(StringIO("()()"), format="newick")
         for i, elem in enumerate(tree.find_clades()):
-            elem.name = f"elem_{i+1}"
+            elem.name = f"elem_{i + 1}"
             elem.width = f"width_{i}"
         graph = Phylo.to_igraph(tree, vertex_attributes=["name"])
         self.assertEqual(graph.vs["name"], ["elem_1", "elem_2", "elem_3"])