Support for newer python syntax

Our team would find it useful if this package could parse an AST from code conforming to the python 3.8 grammar.

I understand that actually supporting newer python features would be a huge project. For my use case, I'm using this project to parse python code to work with the AST but don't need to actually interpret it, which is a much more minor proposition. As far as I know, python 3.8 syntax is completely backwards compatible with 3.4 (avoiding the whole walrus operator thing with python 3.9) and wouldn't even need to alter the internal representation of the AST (much, anyway).

The specific syntax changes our team keeps running into problems with are trailing commas on argument lists in some situations (e.g. after `**kwargs`) and annotations on local or class variables. The former is obviously trivial, the latter still wouldn't need any changes outside the grammar if you don't mind just throwing out the annotation information.

I'll be making these changes for our internal usage; my question is mostly whether there would be interest in a PR to upstream those changes, in which case I might be a bit less "quick and dirty" with my implementation.