feature request: probs arg ignored if openmc.stats.Normal passed to openmc.stats.combine_distributions

[jon-proximafusion]

when making use of openmc.stats.combine_distributions the type hint indicate that probs can be a Sequence[Univariate] but the doc string only mentions openmc.stats.Discrete and openmc.stats.Tabular.

Passing in two openmc.stats.Normal does not raise any errors and returns a Mixture. However the probabilities of the Mixture are set to 1. ,1. even if the users specifies [0.3,0.8]