Add Context-Sensitive Features & Implement Guided Python Generation

[lapp0]

What behavior of the library made you think about the improvement?

Python's syntax isn't context-free due to indentation handling, it is context-sensitive.

It is impossible to create a CFGFSM with Python's production rules.

How would you like it to behave?

Lark provides functionality to extend beyond context-free grammars and can handle a subset of context-sensitive languages, including Python, thanks to lark.PostLex.

Example: https://github.com/lark-parser/lark/blob/master/examples/indented_tree.py#L30

Completion criteria

• Integrate python3.lark https://github.com/lark-parser/lark/blob/master/lark/grammars/python.lark
• Accept optional postlex kwarg in CFGFSM
• Pass post_lex=PythonIndenter() if the grammar has %declare _PYTHON_INDENT
• Leave implementation generic so other PostLex subclasses can be integrated where necessary, e.g. yaml
• Add sample test files to ensure generation is possible