DOC/BLD: remove old sphinx pin
xref https://github.com/pandas-dev/pandas/issues/26058 and https://github.com/pandas-dev/pandas-sphinx-theme/issues/25

We need a newer sphinx version for the new theme. There were some styling issues with the parameter listings on the docstring API pages, but those probably need to redesigned anyway with the new theme.
I am going to merge this, and leave the minimum sphinx version for later. With the new theme development, we still need to check somewhat the minimum sphinx requirements for the sphinx features we are using, so let's revisit this a bit later when having a better idea on this (the theme python package should then also enforce such a minimum version, which is not yet the case).