This was already partly discussed in #9. I think we should start adding some commonly used protocols, type aliases, and other features from _typeshed to typing_extensions so they are available at runtime. This would prevent awkward guarded imports at runtime.

I would suggest to put this into a submodule to separate those types from the "backports and experimental features" that are added to the main namespace. Suggestion: typing_extensions.ext. For a start I would suggest to add the following:

• Some simple dunder protocols: SupportsNext, SupportsAnext, SupportsTrunc, SupportsDivMod, SupportsRDivMod. (I would hold off on protocols like SupportsAdd for now as they are using Any.)
• SupportsGetItem, SupportsItems and SupportsKeysAndGetItem
• The path aliases StrPath, BytesPath. StrOrBytesPath probably doesn't have much value over StrPath | BytesPath.
• Some simple I/O protocols: SupportsRead, SupportsReadline, SupportsWrite