Fix pyproject.toml optional-dependencies (#59)

Declare `optional-dependencies` as dynamic to use the settings from `setup.cfg`.

Closes #58

pyproject.toml

@@ -6,7 +6,7 @@ build-backend = "setuptools.build_meta"
 name = "fiddy"
 description = "Finite difference methods"
 readme = "README.md"
-dynamic = ["version"]
+dynamic = ["version", "optional-dependencies"]
 license = { file = "LICENSE" }
 authors = [
     { name = "The fiddy developers", email = "[email protected]" }