Add upper limit for numpy version in conda recipe

[taprs]

When running skmer from a fresh conda install, I noticed that I have to downgrade numpy to 1.26.4 to make skmer reference run, otherwise (with default numpy=2.0.2 that got installed with the env) I get the following error:

AttributeError: module 'numpy' has no attribute 'math'

I think restricting numpy to <=1.26.4 will solve it for future installations.

[BenKuhnhaeuser]

Same issue here! Of course the cleaner solution would be to update Skmer to work with the latest version of numpy.