Add support for Python 3.8-3.9

[malandrina]

Currently nuanced is compatible with Python >=3.10. It errors when running Python 3.8-3.9.

Steps to reproduce, where VERSION is 3.8 or 3.9:

% pyenv local VERSION
% uv tool uninstall nuanced
% uv tool install ../../nuanced/nuanced-graph
% nuanced init .

Expected result: output saying "Initializing graph ..." and "Done"

Actual result:

py-polars lw-issue-20951 % nuanced init .
Traceback (most recent call last):
  File "/Users/malandrina/.local/bin/nuanced", line 4, in <module>
    from nuanced import cli
  File "/Users/malandrina/.local/share/uv/tools/nuanced/lib/python3.8/site-packages/nuanced/__init__.py", line 1, in <module>
    from nuanced.code_graph import CodeGraph as CodeGraph
  File "/Users/malandrina/.local/share/uv/tools/nuanced/lib/python3.8/site-packages/nuanced/code_graph.py", line 13, in <module>
    class CodeGraph():
  File "/Users/malandrina/.local/share/uv/tools/nuanced/lib/python3.8/site-packages/nuanced/code_graph.py", line 64, in CodeGraph
    def __init__(self, graph:dict|None) -> None:
TypeError: unsupported operand type(s) for |: 'type' and 'NoneType'