Update python-astor to 0.8.0.
- astor.to_source now has a *source_generator_class* parameter to customize source code generation.
- The ~SourceGenerator class can now be imported from the astor package directly. Previously, the astor.code_gen submodule was needed to be imported.
- Fixed string parsing when there is a newline inside an f-string.
- Fixed code generation with escaped braces in f-strings.
- Fixed code generation with attributes of integer literals, and with u-prefixed string literals.
- Fixed code generation with very large integers.
- Fixed astor.tree_walk.TreeWalk when attempting to access attributes created by Python's type system (such as __dict__ and __weakref__)