lil_matrix construction raises FutureWarning (numpy #12609)

Description

Constructing a scipy.sparse.lil_matrix in sparsify_rows raises a Lil Warnin’*

/Users/vl238/miniconda3/envs/gl/lib/python3.7/site-packages/scipy/sparse/lil.py:504: FutureWarning: future versions will not create a writeable array from broadcast_array. Set the writable flag explicitly to avoid this warning.
  if not i.flags.writeable or i.dtype not in (np.int32, np.int64):

Dependency chain:

• call site in sparsify_rows: https://github.com/librosa/librosa/blob/master/librosa/util/utils.py#L1087

• lil_matrix (linked list sparse matrix) class: https://github.com/scipy/scipy/blob/master/scipy/sparse/lil.py#L23

• warning point in scipy.sparse._prepare_index_for_memoryview: https://github.com/scipy/scipy/blob/master/scipy/sparse/lil.py#L504

• scipy PR https://github.com/scipy/scipy/pull/10379

• scipy issue https://github.com/scipy/scipy/issues/10405

• numpy PR https://github.com/numpy/numpy/pull/12609

• numpy issue https://github.com/numpy/numpy/pull/2705

NB: there is another use case of lil_matrix in librosa.segment.recurrence_to_lag. Not sure if it is affected.

Steps/Code to Reproduce

Found during CR of #934.

Versions

Darwin-18.6.0-x86_64-i386-64bit
Python 3.7.3 (default, Mar 27 2019, 16:54:48) 
[Clang 4.0.1 (tags/RELEASE_401/final)]
NumPy 1.17.0
SciPy 1.3.0
librosa 0.7.0

*pun intended: https://www.youtube.com/watch?v=5_Mtgo9WOL0

[bmcfee]

It looks like scipy 1.3.1 has fixed this one for us: https://github.com/scipy/scipy/pull/10379

[lostanlen]

I tried to create a separate conda environment with latest master branches of numpy, scipy, and librosa, in order to see if the warning would disappear. I would guess that it would, in the wake of scipy/scipy#10379. But i did not succeed in installing scipy from source.

 scipy/fft/_pocketfft/pypocketfft.cxx:15:10: fatal error: 'pybind11/pybind11.h' file not found

I don’t know why this happens.