TST: run PEP8 and Pyflakes on the same TravisCI worker.

Saves a little runtime, but more importantly less chance for flaky (no pun
intented) results due to git submodule update failures etc.
MAINT : Returning a warning for low attenuation values of chebwin window
BUG: fix validation in csgraph.shortest_path
BUG: fix logic in shortest_path
BUG: sparse: fix downcast_intp_index to deal with empty arrays
BUG: sparse: avoid introducing 64-bit index arrays spuriously in csr_matrix.__getitem__
Merge pull request #3467 from jakevdp/shortest-path-fix

BUG: fix validation in csgraph.shortest_path
BUG: sparse: try to be more backward-compatible by casting to 32-bit in constructor, if possible

Downstream packages (e.g. scikit-learn 0.13) sometimes construct sparse
matrices using 64-bit index arrays, and assume scipy.sparse casts the
index arrays to 32-bit later on, or, at least, that the 64-bitness
doesn't propagate.

To provide better backward compatibility for them here, we need to cast
integer arrays to int32 in the sparse matrix constructor, when this is
possible.
TST: sparse: test that no spurious 64-bit index arrays pop up

... at least in the base test suite
BUG: sparse: play it safer in bsr_matrix.eliminate_zeros
MAINT : Adding reason in the warning message of chebwin window
DOC: fix docstring for gh-3469
Merge pull request #3462 from ankit-maverick/chebwin

MAINT : Returning a warning for low attenuation values of chebwin window
Merge pull request #3473 from perimosocordiae/patch-3

DOC: improve ndimage.find_objects docstring
FIX: restore np.abs ufunc support for scipy.sparse
STY: sparse: rewrite call to abs in __numpy_ufunc__ in a different way
Merge pull request #3470 from pr-3470

BUG: sparse: restore np.abs ufunc support for scipy.sparse
BUG: sparse/_csparsetools: cast integer indices to same dtype
Merge pull request #3468 from pv/sparse-64fix

BUG: sparse: fix backward compatibility issues in scipy.sparse vs 64-bit indices
MAINT: fix cythonize treatment of .pyx.in files; use Cython-bundled Tempita