fix: replace ast.Str and drop astor

[lafirm]

Description

Closes #5837

• Updated minimum Python requirement from 3.8 to 3.9 in documentation to match with the pyproject.toml
• Removed astor from dependencies and mypy overrides in pyproject.toml, using ast.unparse introduced in Python 3.9
• Replaced all astor.to_source calls with ast.unparse in sqlmesh/core/model/common.py and sqlmesh/utils/metaprogramming.py

Test Plan

• Updated test expectations in tests/utils/test_metaprogramming.py to match the output format of ast.unparse.

Checklist

• I have run make style and fixed any issues
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test)
• My commits are signed off (git commit -s) per the DCO