Bump typed_ast 1.4.2 -> 1.4.3 (#109)

To get the fix for python/typed_ast#159; With
1.4.2 `typed_ast` can't be installed on recent Pythons. Note that this
 new version is still compatible with the version specifications in
`pyproject.toml`; this is "just" a `poetry update typed_ast`.

Side note: `typed_ast` is going away next July: python/typed_ast#179

Author: abesto