Mypy: Update fastparse for 3.9 AST changes

[msullivan]

In 3.9.0a5, some changes landed to the AST for subscripting (https://github.com/python/cpython/pull/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.

[jaraco]

I'm attempting to test against Python 3.9b1, but the test suite is failing with the "syntax error in type comment". Does this patch fix it? Is a release planned?

[JelleZijlstra]

@jaraco on another issue Jukka said that another release is planned in a week or so.

[jaraco]

I see v0.780 includes the fix for the issue.