Python 3.9 no longer installable: highdicom requires pydicom>=3.0.1 which requires Python>=3.10 #1918

Problem

The deps_check matrix in .github/workflows/pythonapp.yml includes Python 3.9, but dependency resolution now fails on it:

ERROR: Could not find a version that satisfies the requirement pydicom==3.0.1
ERROR: No matching distribution found for pydicom==3.0.1

Root cause is a transitive pin:

• requirements.txt pins highdicom==0.26.1 and pydicom==3.0.1.
• highdicom==0.26.1 requires pydicom>=3.0.1.
• pydicom 3.0.x requires python>=3.10.

So Python 3.9 can no longer be installed with this dependency set, and the matrix must drop 3.9 (or highdicom/pydicom must be pinned to older, 3.9-compatible versions).

Suggested fix

Drop "3.9" from the deps_check, build, and packaging matrices in .github/workflows/pythonapp.yml (and update any other declared 3.9 support), or pin a 3.9-compatible highdicom/pydicom pair if 3.9 support is still required.