DOC: update conf.py to use new numpy doc url
- [X] closes #33953
- [X] tests added / passed
- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Thanks @codeloop, did you build the docs locally and checked that the link is working all right? I guess the intersphinx file is available at the new URL, but we need to make sure before merging this. Thanks! @datapythonista , I'm building the docs, following the steps from here [Contributing to the documentation](https://pandas-docs.github.io/pandas-docs-travis/development/contributing.html#contributing-to-the-documentation), will update after build is complete & tested locally. successfully built documentation locally using this commit, now the links point to numpy URL. 
For eg: In the pandas docs at https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Series.argsort.html
earlier it was referencing to : https://docs.scipy.org/doc/numpy/reference/generated/numpy.ndarray.argsort.html#numpy.ndarray.argsort
After this commit & build it references to : https://numpy.org/doc/stable/reference/generated/numpy.ndarray.argsort.html#numpy.ndarray.argsort

Looks good to me. @jorisvandenbossche may be you want to have a look @Meeseeksdev backport to 1.0.x

