delete requirements.txt, prune unneeded requirements, and consolidate env/*.yml into exoctk-env.yml

[zacharyburnett]

the separate env/ files for each Python version are not necessary, nor is the duplication of all the requirements in requirements.txt. I also pruned the requirements down to only what is explicitly imported in exoctk and in the notebooks, and moved sphinx, pytest, and ipython to the docs extra, tests extra, and Conda environment respectively. This should significantly reduce the complexity of the actual package environment.

[zacharyburnett]

a single test is failing at https://github.com/ExoCTK/exoctk/actions/runs/16053688458/job/45302531745?pr=647#step:7:47

>           assert data[value] == pytest.approx(planet_data[value], 0.1)
E           assert -0.05 == -0.01 ± 0.001
E             
E             comparison failed
E             Obtained: -0.05
E             Expected: -0.01 ± 0.001

I don't think this is related to the changes in this PR