Update pyproject.toml

pyproject.toml

 [build-system]
-requires = ["setuptools", "cython"]
+requires = [
+  "setuptools", 
+  "cython"
+  ]
 build-backend = "setuptools.build_meta"
 
 [project]
@@ -52,7 +55,6 @@ dependencies = [
 ext-modules = [
   {name = "tetrax.sample.mesh.fem_core.cythoncore",
   sources = [
-      "tetrax/sample/mesh/fem_core/cythoncore.pyx",
       "tetrax/sample/mesh/fem_core/dense2D_onthefly.c",
       "tetrax/sample/mesh/fem_core/bessel_K1.c",
       "tetrax/sample/mesh/fem_core/akmag.c",