Support partition weights and constraints

The PyMetis provides a 'part_graph' like this:

```
def part_graph(nparts, adjacency=None, xadj=None, adjncy=None,
vweights=None, eweights=None, recursive=None, contiguous=None, options=None):
```

And I notice the original METIS in C provides the k-way partition function like this:

```
int METIS_PartGraphKway(idx_t *nvtxs, idx_t *ncon, idx_t *xadj, idx_t *adjncy,
idx_t *vwgt, idx_t *vsize, idx_t *adjwgt, idx_t *nparts,
real_t *tpwgts, real_t *ubvec, idx_t *options, idx_t *objval, idx_t *part)
```

According to the manual, the parameter 'tpwgts' specifies the weights for each partition, but seems missing in current API.

Start with the part_graph API shown in the issue and compare its arguments with METIS_PartGraphKway and the METIS manual, especially tpwgts and the constraint-related parameters. Done means the PyMetis interface clearly exposes the requested partition weights and constraints with behavior matching the underlying API.

Written by the indexing model from the issue text.