Format code with black

This commit fixes the style issues introduced in e7ff6db according to the output
from black.

Details: https://deepsource.io/gh/MilesCranmer/PySR/transform/ddcd62c1-7279-4f35-8952-ba67296304e2/

pysr/sr.py

@@ -952,8 +952,6 @@ class PySRRegressor(BaseEstimator, RegressorMixin):
                 "If you are using a custom operator, make sure to define it in extra_sympy_mappings, "
                 "e.g., `model.set_params(extra_sympy_mappings={'inv': lambda x: 1 / x})`."
             ) from error
-                
-            
 
     def sympy(self, index=None):
         """Return sympy representation of the equation(s) chosen by `model_selection`.