BUG: special: pbvv_seq is broken.

[WarrenWeckesser]

The function scipy.special.pbvv_seq fails to run:

In [1]: import scipy

In [2]: scipy.__version__
Out[2]: '1.11.0.dev0+1208.2af3085'

In [3]: from scipy.special import pbvv_seq

In [4]: pbvv_seq(3, 1.5)
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In [4], line 1
----> 1 pbvv_seq(3, 1.5)

File ~/repos/git/forks/scipy/build-install/lib/python3.10/site-packages/scipy/special/_basic.py:2235, in pbvv_seq(v, x)
   2233     n1 = n
   2234 v1 = n1 + v0
-> 2235 dv, dp, pdf, pdd = _specfun.pbvv(v1, x)
   2236 return dv[:n1+1], dp[:n1+1]

AttributeError: module 'scipy.special._specfun' has no attribute 'pbvv'

This function has no tests, otherwise we would have caught this issue a long time ago. I don't know how long it has been broken. If it has never worked, we could consider simply deleting it. Edit: pbvv_seq was discussed in #4957, so apparently it was working back in 2015.

[tupui]

+1 for the deletion if it's not working since a few releases. If no one is complaining, probably not used.

[WarrenWeckesser]

After reading #4957, it is clear that pbvv_seq has worked in the past. It broke some time after that issue was reported in 2015.

[steppi]

It seems the wrapper for pbvv was inadvertently removed in #11740, and this went unnoticed because it wasn't tested. We can just put it back (and anything else which shouldn't have been removed) and add some tests.