Drop support for Python 3.8 and 3.9; test on 3.13 and 3.14

[mdickinson]

Raise the minimum supported Python to 3.10 and extend the test matrices
upwards.

What changed

• python_requires is now >=3.10; test-with-pip.yml runs 3.10–3.14
  and test-minimal-dependencies.yml runs 3.10–3.15. 3.15 is at rc.2, so
  setup-python needs allow-prereleases: true to resolve it; the flag has no
  effect on the other entries, and it stays in place so later versions can be
  added to the matrix as soon as they reach beta. 3.15 is confined to the
  minimal workflow because the h5 extra pulls in tables → blosc2 →
  ndindex, and ndindex has no cp315 wheel and segfaults building from its
  sdist.
• EDM ships no runtime newer than 3.11, so test-with-edm.yml and
  ets-from-source.yml now build only a 3.11 environment, and
  etstool.py's per-runtime dependency split collapses into a single
  EDM_DEPENDENCIES set. That also fixes the weekly
  "Test with EDM using ETS packages from source"
  failures: traits' main branch has required Python >= 3.10 since
  Require Python >= 3.10, and replace pkg_resources with importlib.resources traits#1895, so installing it into a 3.8 environment has failed
  every week since 2026-08-14.
• importlib_resources is gone as a fallback for
  importlib.resources.files, along with the test extra that supplied it.
• The numpy < 2.0; python_version<'3.10' markers are gone — the PyTables
  incompatibility they worked around (PyTables is incompatible with NumPy 2.0 #345) only applied below 3.10.
• The macOS-only install split in test-with-pip.yml is gone; it existed
  because PyTables would not build on Apple Silicon for Python < 3.10 (Test workflows fail on macOS due to PyTables #344).

One thing worth a look

flake8_ets is not packaged in EDM for 3.11 on any platform, so the EDM style
check no longer runs the H101/H102 copyright checks. check-style-strict.yml
is untouched and still checks copyright end years with flake8-ets from PyPI;
H101 is no longer enforced anywhere.

Verification

Locally, on 3.10 and 3.14, pip install .[gui,h5,preferences] resolves and all
358 tests pass; on 3.15, pip install .[test] resolves against traits 7.2.0's
cp315 wheel and the suite passes. flake8 over the usual targets is clean both
with and without flake8-ets. Every package in the new EDM_DEPENDENCIES set is
available in EDM for 3.11 on both rh8-x86_64 and win-x86_64.

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