gnu: python-plotly: Update to 4.8.1.

But keep version 2.4.1 around for packages that still need it and for python2
support.

* gnu/packages/graph.scm (python-plotly): Update to 4.8.1.
[source]: Use git-fetch so that tests are also included.
[arguments]: Enable tests.
[home-page]: Follow redirect.
(python-plotly-2.4.1): New variable.
(python2-plotly): Inherit from python-plotly-2.4.1.
* gnu/packages/bioinformatics.scm (python-deeptools)[native-inputs]: Depend on
python-plotly-2.4.1.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>