BUG: special: Fix two XSLOW test failures. (#17190)
BUG: special: Fix two XSLOW test failures. (#17190)

When SCIPY_XSLOW is defined, the tests `test_riemann_zeta` and `test_zetac`
(that compare the results of `zeta` and `zetac` to the results computed
with `mpmath`) are run with 5000 points, and it turns out that the input
value 1 is included in that case.  The SciPy functions returns `inf`, but
`mpmath.zeta(1)` raises an exception, and that caused the tests to fail.

The fix is to wrap the `mpmath` function to return `mpmath.inf` when the
input is 1.  After fixing that, both tests then failed because some inputs
result in a relative error of roughly 2e-13, so I bumped the tolerance up
to 5e-13.
Stefan-Endres/scipy,Stefan-Endres/scipy,ilayn/scipy,ilayn/scipy,Stefan-Endres/scipy,ilayn/scipy,scipy/scipy,andyfaff/scipy,scipy/scipy,mdhaber/scipy,endolith/scipy,mdhaber/scipy,andyfaff/scipy,endolith/scipy,mdhaber/scipy,endolith/scipy,anntzer/scipy,Stefan-Endres/scipy,mdhaber/scipy,ilayn/scipy,Stefan-Endres/scipy,anntzer/scipy,WarrenWeckesser/scipy,andyfaff/scipy,mdhaber/scipy,endolith/scipy,andyfaff/scipy,WarrenWeckesser/scipy,andyfaff/scipy,Stefan-Endres/scipy,WarrenWeckesser/scipy,scipy/scipy,scipy/scipy,anntzer/scipy,anntzer/scipy,anntzer/scipy,ilayn/scipy,endolith/scipy,endolith/scipy,scipy/scipy,WarrenWeckesser/scipy,mdhaber/scipy,WarrenWeckesser/scipy,scipy/scipy,anntzer/scipy,WarrenWeckesser/scipy,ilayn/scipy,andyfaff/scipy