Reverse the pin on Cython.

Author: Shyue Ping Ong

pymatgen/util/coord_cython.pyx

@@ -91,7 +91,7 @@ def pbc_shortest_vectors(lattice, fcoords1, fcoords2, mask=None, return_d2=False
 
     pbc = lattice.pbc
     cdef int n_pbc = sum(pbc)
-    cdef int n_pbc_im = int(round(3 ** n_pbc))
+    cdef int n_pbc_im = int(3 ** n_pbc)
 
     cdef np.float_t[:, ::1] frac_im
     cdef int i, j, k, l, I, J

pyproject.toml

@@ -1,6 +1,6 @@
 [build-system]
 requires = [
-  "Cython>=0.29.23,<3",
+  "Cython>=0.29.23",
   # pin NumPy version used in the build
   "oldest-supported-numpy",
   "setuptools>=65.0.0",