BUG: Fix writing of intrinsic long integers in python2 npy files.

This only happens when C long is smaller than npy_intp, in particular,
on 64 bit windows. The 'L' suffix is not needed as long as safe_eval
is used to recover the values, and the long intrinsic causes problems
for python3.

The python3 read side has been fixed, but we might as well fix the
problem at the root as well.
Merge pull request #5183 from charris/fix-npy-header-write

BUG: Fix writing of intrinsic long integers in python2 npy files.