[SCRIPT] Bump Python minimum version to 3.9 and update AST compatibility

[Hzfengsy]

• Set Python minimal version to 3.9 as Python 3.8 has reached end of life
• Update AST handling to match Python 3.9+ API changes
• Fix deprecation warning: "Support for arbitrary keyword arguments is deprecated and will be removed in Python 3.15"
• Update TVMScript parser components to use modern AST interfaces
• Adjust linting and test infrastructure for Python 3.9+ compatibility

[Copilot]

Pull Request Overview

This PR bumps the minimal Python version to 3.9 and updates the AST handling to align with changes in Python 3.9+ while removing legacy compatibility branches. Key changes include:

• Updating conditionals and error messages in various scripts and test files for Python 3.9+.
• Removing legacy AST helper functionality and adjusting context assignments in AST node processing.
• Simplifying Windows DLL directory setup in light of updated version requirements.

Reviewed Changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/scripts/ci.py	Removed legacy Python 3.8 branches in AST helper functions.
tests/python/relax/test_tvmscript_parser.py	Removed outdated skipif decorators for Python versions below 3.8.
tests/lint/filter_untracked.py	Removed Python 2 compatibility branch.
python/tvm/script/parser/tir/parser.py	Updated AST node context creation to use parameterless doc.Load/Store.
python/tvm/script/parser/core/evaluator.py	Adjusted type-checking to match new API naming conventions.
python/tvm/script/parser/core/doc.py	Removed legacy registration functions now unnecessary for 3.9+.
python/tvm/base.py	Updated Python version check and modified Windows DLL directory logic.

Comments suppressed due to low confidence (3)

python/tvm/script/parser/tir/parser.py:286

• Review the removal of positional arguments in doc.Load() calls to ensure that no critical diagnostic context is lost for downstream AST processing.

    node.target.ctx = doc.Load()

python/tvm/base.py:42

• Since the DLL search path logic is now applied unconditionally on Windows, please confirm that this change aligns with the intended behavior for Python 3.9+ environments.

    if sys.platform.startswith("win32"):

python/tvm/script/parser/core/doc.py:321

• Legacy AST registration functions have been removed; verify that downstream modules no longer depend on any of these removed handlers.

_register_default()

[Hzfengsy]

cc @tqchen