Python 3.8 support for macro code

[Technologicat]

In the Python 3.8 AST, all constants are now represented by the ast.Constant node type.

Thus to support 3.8 and later properly, we will need to update any macro code that deals with Num, Str, NameConstant.

Maybe in 0.15, move these into syntax.astcompat, and start accepting Constant nodes as an alternative.

There are not many use sites, so it's probably not worth building an abstraction for this.

[Technologicat]

Related to #1.

[Technologicat]

Issues cleanup: 0.15 is probably a long way off; we should actually add 3.8 support earlier. Maybe in a small service release after 0.14.2?

[Technologicat]

The GTS docs have updated to 3.8.

[Technologicat]

This is a bug in that until we support ast.Constant, the macros won't work right on Python 3.8.

[Technologicat]

0.14.3 is becoming such a big update already, maybe it's better to add Python 3.8 support in the next release.

[Technologicat]

Also consider if we need to do something with ast.NamedExpr introduced in 3.8.

[Technologicat]

While at it, we should add support also for Python 3.9.

The ast.Index wrapper in ast.Subscript is gone, and ast.ExtSlice has changed a bit. See Technologicat/mcpyrate#20.

[Technologicat]

Done. Now just need to port the macro code to mcpyrate (which is orthogonal to the AST changes).