cleanup the imports of Eta products in global namespace

[chapoton]

part of #25383

[chapoton]

New commits:

​c69ce6e

cleanup and deprecate imports for Eta products

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​b000350

fix

[git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

​824d3fe	cleanup and deprecate imports for Eta products
​8c57b11	fix : adding one missing import in doc

[chapoton]

green bot, this is ready for review.

[vdelecroix]

Because you touched it, can you make the assert a ValueError? assert is not intended to check user input at all.

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​230d71f

using raise instead of assert to check prec input of eta expansion

[chapoton]

done, thanks