distinguish "doesn't exist" from "doesn't exist on this Python version"

[carljm]

If a user has their Python version mis-configured, and gets an error like module 'datetime' has no attribute 'UTC', it's not necessarily obvious that the reason for this is that they are on the wrong Python version. It would be nice if we could offer an info context on the diagnostic saying "it exists on other Python versions, but you are running with Python 3.9"

We have basic support for this, but there's more we could do. Quoting a comment below: We now:

• Report this precise info when you import a stdlib module we have VERSIONS info for (18403, 20908)
• Mention what version you're on when you fail to access an attribute on a stdlib module we believe does exist in some version/platform (20909)

We do not:

• Report on attributes of types, optional function arguments, etc.
• Report the exact version(s) needed for attributes
• Report the exact platform(s) needed for attributes (or mention the current platform)

[MichaReiser]

Yeah, that would be awesome. But it might be somewhat involved to implement the detection.

[carljm]

An easy initial step here would just be to always alert the user to their detected Python version, if an import from stdlib fails.

[Gankra]

We now:

• Report this precise info when you import a stdlib module we have VERSIONS info for (18403, 20908)
• Mention what version you're on when you fail to access an attribute on a stdlib module we believe does exist in some version/platform (20909)

We do not:

• Report on attributes of types, optional function arguments, etc.
• Report the exact version(s) needed for attributes
• Report the exact platform(s) needed for attributes (or mention the current platform)

[Gankra]

If we want to leave this open for further improvements, I think we can downgrade this to GA.