Test the from_networkx int32 widening and correct its coverage claim - #618

The widening added while stabilizing CI was marked "pragma: no cover
(win64 numpy < 2)", but the branch is reachable on every platform:
np.int32 scalar edge weights infer an int32 array from the edge sequence
anywhere, not only where python ints do. The pragma therefore hid live
code from coverage, and the comment implied the change was platform-local
when it is user-visible everywhere: int32-weighted graphs that previously
round-tripped through scipy as INT32 now come back INT64.

The widening itself is kept, deliberately. from_coo widens an int32 dtype
inferred from sequence input to int64 on every platform
(values_to_numpy_buffer), and from_networkx builds its values from a
Python sequence, so identical weights now produce the same dtype through
either constructor. Gating the widening to win64 instead would have
re-introduced a from_coo/from_networkx disagreement everywhere else.

Drop the pragma, restate the comment in terms of the real contract, and
pin the behavior: np.int32 weights widen to INT64 and agree with from_coo,
an explicit dtype= is preserved exactly, and np.int16 stays INT16 through
both constructors. Release-note item: the from_networkx result dtype for
int32-weighted graphs changes from INT32 to INT64.