Fix pandas intersphinx mapping; fix test setup

[stes]

In #135 , the tests show a few errors that are due to upstream changes in our dependencies. This PR fixes the build errors:

• Update the URL of the pandas intersphinx mapping to the current stable version (vs. the dev version, which we used previously). This fixes the issue during docs build due to a missing reference.
• Pin pytest and pytest-sphinx to a previous release --- we use pytest-sphinx, which is not yet compatible with the latest pytest version.

[stes]

Confirmed, can repro error from #135 :

[stes]

Upstream issue seems to be this line (https://github.com/thisch/pytest-sphinx/blob/2d995d2a4bb59d6e09610115740a9da2bc3d0c36/src/pytest_sphinx.py#L560), potentially due to API change in Pytest (https://pypi.org/project/pytest/#history) version 8.

[stes]

updated the intersphinx path to pandas, the prev version already contained the dev preview:

likely this caused the error