IIUC
pd.cut(pd.Series(x), bins=range(0,int(6e9),int(1e9)))
Out[392]:
0 (1000000000, 2000000000]
1 (1000000000, 2000000000]
2 (2000000000, 3000000000]
3 (2000000000, 3000000000]
4 (3000000000, 4000000000]
5 (4000000000, 5000000000]
dtype: category
Categories (5, interval[int64]): [(0, 1000000000] < (1000000000, 2000000000] <
(2000000000, 3000000000] < (3000000000, 4000000000] <
(4000000000, 5000000000]]