ci: python version coverage

[JPHutchins]

Drop 3.6 and 3.7 support. Update CI with coverage for all supported versions, including those that are EOL (3.8, 3.9), and near EOL (3.10).

Wrote a script to test locally:

scripts/tests/local-matrix.sh
All requested versions already installed

========== Python 3.8 ==========
....................................................................................... [ 50%]
.....................................xxxx..x.................x........................x [100%]
167 passed, 7 xfailed in 6.73s
========== 3.8 OK ==========

========== Python 3.9 ==========
....................................................................................... [ 50%]
.....................................xxxx..x.................x........................x [100%]
167 passed, 7 xfailed in 5.86s
========== 3.9 OK ==========

========== Python 3.10 ==========
....................................................................................... [ 50%]
.....................................xxxx..x.................x........................x [100%]
167 passed, 7 xfailed in 6.69s
========== 3.10 OK ==========

========== Python 3.11 ==========
....................................................................................... [ 50%]
.....................................xxxx..x.................x........................x [100%]
167 passed, 7 xfailed in 5.88s
========== 3.11 OK ==========

========== Python 3.12 ==========
....................................................................................... [ 50%]
.....................................xxxx..x.................x........................x [100%]
167 passed, 7 xfailed in 6.45s
========== 3.12 OK ==========

========== Python 3.13 ==========
....................................................................................... [ 50%]
.....................................xxxx..x.................x........................x [100%]
167 passed, 7 xfailed in 6.14s
========== 3.13 OK ==========

========== Python 3.14 ==========
....................................................................................... [ 50%]
.....................................xxxx..x.................x........................x [100%]
167 passed, 7 xfailed in 6.59s
========== 3.14 OK ==========

========== Python pypy3.9 ==========
....................................................................................... [ 50%]
.....................................xxxx..x.................x........................x [100%]
167 passed, 7 xfailed in 13.73s
========== pypy3.9 OK ==========

========== Python pypy3.10 ==========
....................................................................................... [ 50%]
.....................................xxxx..x.................x........................x [100%]
167 passed, 7 xfailed in 14.63s
========== pypy3.10 OK ==========

========== Python pypy3.11 ==========
....................................................................................... [ 50%]
.....................................xxxx..x.................x........................x [100%]
167 passed, 7 xfailed in 9.37s
========== pypy3.11 OK ==========

All versions passed.

[Copilot]

Pull request overview

Updates the imgtool packaging metadata and CI to reflect dropping Python 3.6/3.7 support and to exercise the test suite across a broader set of supported Python runtimes.

Changes:

• Bump python_requires for imgtool to >=3.8.
• Replace Python 3.10+ union-type syntax in isinstance() with a tuple form compatible with Python 3.8/3.9.
• Expand the GitHub Actions test matrix to run across multiple CPython and PyPy versions (and update referenced action major versions).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
scripts/setup.py	Raises minimum supported Python version for the package to 3.8+.
scripts/imgtool/keys/general.py	Avoids X | Y union syntax so the code runs on Python 3.8/3.9.
.github/workflows/imgtool.yaml	Expands CI matrix to cover multiple CPython/PyPy versions and updates action references.

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