Format code with black (#195)

This commit fixes the style issues introduced in 9b49654 according to the output
from black.

Details: https://deepsource.io/gh/MilesCranmer/PySR/transform/b2ff6f04-c0ef-4ccc-bfa0-0761146d8e22/

Co-authored-by: deepsource-autofix[bot] <62050782+deepsource-autofix[bot]@users.noreply.github.com>

example.py

@@ -14,7 +14,7 @@
         "exp",
         "sin",
         "inv(x) = 1/x",
-	# ^ Custom operator (julia syntax)
+        # ^ Custom operator (julia syntax)
     ],
     extra_sympy_mappings={"inv": lambda x: 1 / x},
     # ^ Define operator for SymPy as well