Update fastparse for 3.9 AST changes

[msullivan]

In 3.9.0a5, some changes landed to the AST for subscripting (python/cpython#9605). We need to support them.

This first got reported in #8614 as "syntax error in type comment", which is pretty bad, also! We should clean up that error message when we fix this.

[cooperlees]

If it helps, I've got CI failing 3.9.0a5 here with the same error: https://travis-ci.org/github/cooperlees/aioexabgp/jobs/673829413

• If not, please ignore.
• Happy to help test anything if needed

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[gvanrossum]

Do you need help with this? I reviewed the CPython change and I can probably figure out how to do this easily.