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

[StuffbyYuki]

@lafirm Can you check the failed ci test? Also, do you think it's worth considering a brief changelog/release note: python_env payloads feed model data_hash, so users with Python macros may see a one-time fingerprint change on upgrade (e.g. lambda : → lambda:)?

[lafirm]

Migration note:

Users with Python models or Python macros will have their snapshots automatically re-fingerprinted on sqlmesh migrate. No backfills are triggered. This is a one-time side-effect of replacing astor with ast.unparse for Python source normalisation.

[lafirm]

@lafirm Can you check the failed ci test? Also, do you think it's worth considering a brief changelog/release note: python_env payloads feed model data_hash, so users with Python macros may see a one-time fingerprint change on upgrade (e.g. lambda : → lambda:)?

sure, I've added a file to migrate the existing rows - hopefully that fixes the CI test. Please let me know where do you want to have this change log (though I already have left a comment in this PR and added a doc string in the migration file)