Adopt general-purpose-identifiers as dataset names

[mattijn]

Could we adopt general-purpose-identifiers according to UAX-31, for the dataset names? In that case the dataset names are also valid python identifiers.

Eg, the dataset name 7zip is invalid as it starts with a digit. Also, one can argue that the dataset name 7zip is a bit misleading, it's not that we include the application itself, so changing it to logo_7zip would improve the name of the "dataset" and makes it also a valid python identifier.

[mattijn]

Suggestion originally raised in vega/altair#3631 (comment)

[dangotbanned]

Some extra context, since the PR may be easy to get lost in 😄:

• feat(RFC): Adds altair.datasets altair#3631 (comment)
• feat(RFC): Adds altair.datasets altair#3631 (comment)
• feat(RFC): Adds altair.datasets altair#3631 (comment)

[dsmedia]

On a practical level, actual usage of the 7zip png as a "dataset" seems mostly confined to an official scatter_image documentation example. This suggests the impact of the breaking change would be minimal and primarily internal (updating the docs), making it a very manageable fix.

More broadly, establishing a consistent naming convention feels like a good investment for the dataset collection's future for the sake of consistency and predictability.