MAINT remove harmless, but useless, double call to csr_matrix

The input validation code will already have performed the conversion.
Calling this constructor on a CSR matrix doesn't actually copy, but
it's useless too.
MAINT use %r for better printing of regexps
ENH friendlier message for calling predict before fit on SVMs

Fixes #3601.
ENH Add 'axis' argument to sparsefuncs.mean_variance_axis
ENH improved CSC matrix handling in VarianceThreshold
FIX heisenfailure in test_lasso_lars_path_length

Tentative fix for #3370: random failure under Python 32 bit and
Windows.
Merge pull request #3629 from ogrisel/fix-unstable-lars-test

[MRG+1] FIX heisenfailure in test_lasso_lars_path_length