Backports upstream fix for CVE-2023-50447 (arbitrary code execution via builtins/dunder keys in ImageMath.eval env).

Source

• Upstream PR: Restricted environment keys for ImageMath.eval() python-pillow/Pillow#7655
• Cherry-picked commits:
  • 45c726fd4daa63236a8f3653530f297dc87b160a — Don't allow __ or builtins in env dictionaries for ImageMath.eval
  • 0ca3c33c59927e1c7e0c14dbc1eea1dfb2431a80 — Allow ops
  • 557ba59d13de919d04b3fd4cdef8634f7d4b3348 — Include further builtins

Changes

• ImageMath.eval now raises ValueError when a user-supplied env key contains __ or matches a name on the builtins module.
• Validation runs only over _dict and kw keys (not ops).
• New tests test_prevent_double_underscores and test_prevent_builtins in Tests/test_imagemath.py.
• Release notes added to new docs/releasenotes/9.5.0.1.rst (upstream's 10.2.0.rst update was dropped — not applicable to 9.5.x).

Test plan

• pytest Tests/test_imagemath.py