add pyensmallen to config.py

Author: gauravmanmode

src/optimagic/config.py

@@ -39,6 +39,7 @@ def _is_installed(module_name: str) -> bool:
 IS_IMINUIT_INSTALLED = _is_installed("iminuit")
 IS_NEVERGRAD_INSTALLED = _is_installed("nevergrad")
 IS_BAYESOPT_INSTALLED = _is_installed("bayes_opt")
+IS_PYENSMALLEN_INSTALLED= _is_installed("pyensmallen")
 
 
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