Fix deprecation warning from pkg_resources in sphstat.twosample

[Vlad-Kor]

Importing sphstat currently prints the following warning:

/home/vlad/.cache/pypoetry/virtualenvs/web-app-Dk6Rue85-py3.10/lib/python3.10/site-packages/sphstat/twosample.py:59: UserWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html. The pkg_resources package is slated for removal as early as 2025-11-30. Refrain from using this package or pin to Setuptools<81.
  import pkg_resources as pkg

This eliminates the warning, by using importlib.resources instead of pkg_resources.

tested by calling:

import sys
sys.path.insert(0, 'src')
from sphstat.twosample import a20, a21, a23
print('a20:', a20(N=12, alpha=0.05, Rbar=0.7))
print('a21:', a21(gamma=2, N=20, alpha=0.05, Rbar=0.1))
print('a23:', a23(nu=2, r=2))

a20: 0.793
a21: 0.355
a23: 39.0