Add toml module as explicit requirement

black recently dropped this requirement
(psf/black#2301), so this results in an error if
this is run with a newer version of black.

setup.py

@@ -26,7 +26,7 @@ def get_long_description() -> str:
     py_modules=["black_nbconvert"],
     python_requires=">=3.6",
     zip_safe=False,
-    install_requires=["black", "nbconvert"],
+    install_requires=["black", "nbconvert", "toml"],
     classifiers=[
         "Development Status :: 4 - Beta",
         "Environment :: Console",