fix Python 3.8 ast export

[TiberiusNemesis]

Fixes #192

now export ast was passing indent=2 to ast.dump, but that kwarg only exists on Python 3.9+. the package still allows Python 3.8, so this keeps the current version floor.

so theses change basically:

• use indented AST output on Python 3.9+
• use plain ast.dump on Python 3.8
• add focused tests for both branches