Format code with black

This commit fixes the style issues introduced in 816c213 according to the output
from black.

Details: https://deepsource.io/gh/MilesCranmer/PySR/transform/e57b6fef-eefc-4d29-bb6a-4b461cba6c8e/

docs/generate_papers.py

@@ -57,4 +57,4 @@ with open(output_file, "w") as f:
 """
         snippets.append(paper_snippet)
 
-    f.write("\n\n---\n\n".join(snippets))
+    f.write("\n\n---\n\n".join(snippets))

pysr/sr.py

@@ -1036,7 +1036,7 @@ class PySRRegressor(MultiOutputMixin, RegressorMixin, BaseEstimator):
     def _setup_equation_file(self):
         """
         Set the full pathname of the equation file.
-        
+
         This is performed using `tempdir` and
         `equation_file`.
         """
@@ -1678,7 +1678,7 @@ class PySRRegressor(MultiOutputMixin, RegressorMixin, BaseEstimator):
     def refresh(self, checkpoint_file=None):
         """
         Update self.equations_ with any new options passed.
-        
+
         For example, updating `extra_sympy_mappings`
         will require a `.refresh()` to update the equations.