Updates to support python 3.9 AST

[iamdefinitelyahuman]

What I did

Update the vyper.ast subpackage to handle changes introduced in the python AST in version 3.9.

Related to #2182
Closes #2205
Closes #2206

How I did it

The only change in 3.9 relevant to Vyper was the deprecation of the Index node class:

Deprecated since version 3.9: Old classes ast.Index and ast.ExtSlice are still available, but they will be removed in future Python releases. In the meantime, instantiating them will return an instance of a different class.

This node is exclusively used within slice attribute of the Subscript node - starting in 3.9, slice now contains Index.value directly.

I had to get hacky to sidestep the logic that prevents direct instantiation of Index. It works, I've explained it in the docstring, but this is a great example of why we really need our own parser.

How to verify it

We're unable to run the test suite against python 3.9 in the CI, because of a lack of support from an upstream dependency: ethereum/blake2b-py#2

I've run the tests locally and verified that everything is passing. I think we should merge this PR now, and once our test deps support 3.9 we can announce official support.

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