fix: add missing numdifftools dependency

Co-authored-by: derrynknife <18457108+derrynknife@users.noreply.github.com>

Author: google-labs-jules[bot]

requirements.txt

@@ -4,4 +4,5 @@ lifelines==0.27.4
 numba==0.56.4
 numpy-indexed==0.3.5
 reliability==0.8.6
-matplotlib==3.6
+matplotlib==3.6
+numdifftools

setup.py

@@ -30,6 +30,7 @@
         "numpy_indexed",
         "numba",
         "formulaic",
+        "numdifftools",
     ],
     include_package_data=True,
     package_data={"": ["datasets/*.csv", "py.typed"]},