BUG: sparse: fix operator overloading with custom types

See issue #3580.
ENH: sparse: inplace ops should fall back to x = x op y if not implemented
BUG: sparse: itruediv should be allowed on sparse matrix
TST: speed up TravisCI tests of scipy.special, by installing gmpy2.

Gain is ~230 seconds on my machine (will be more on TravisCI).
DOC: remove `special.jn` from refguide and mention it's an alias of jv.

Closes gh-2564.
Merge pull request #3681 from rgommers/speedup-tests

MAINT: install gmpy2 in travis-ci to speed up tests
Merge pull request #3589 from larsmans/sparse-binop

BUG: sparse: allow operator overloading

Make binop methods should return NotImplemented rather than raising errors.
DOC: add some docstrings for special functions in the nctdtr family.

These were not yet in the built docs, see gh-2746.
DOC: add some docstrings for special functions in the ncfdtr family.

These were not yet in the built docs, see gh-2746.
DOC: add docstrings for special.nrdtrisd/nrdtrimn.
GEN: regenerate special/_ufuncs.pyx
DEP: deprecate special.bessel_diff_formula, it's a private function.
DOC: add docstring to special.assoc_laguerre and add that to built docs.

Closes gh-2746.
Merge pull request #3684 from rgommers/special-docs

Update special docs
DOC: spacing in special docstrings
GEN: regenerate special/_ufuncs.pyx
Merge pull request #3688 from ev-br/special_spaces

Spacing in special docstrings