Defer spython imports in singularity.py to where they're used

[markjschreiber]

Summary

cwltool/singularity.py imports spython.main.Client, spython.main.parse.parsers.docker.DockerParser, and spython.main.parse.writers.singularity.SingularityWriter at module level. Since cwltool.workflow/cwltool.command_line_tool import cwltool.singularity unconditionally, this means spython (MPL-2.0 licensed) must be installed just to import cwltool at all — even for use cases like cwltool --validate that never touch Singularity.

All three names are only actually used inside SingularityCommandLineJob.get_image's Dockerfile-build branch (building a Singularity image from a dockerFile requirement), never at import time. This PR moves the three imports into that branch, right before first use.

No behavior change for actual Singularity usage — the imports still happen, just lazily, the first time an image actually needs to be built from a Dockerfile.

Verification

• flake8, black --check, and mypy all pass on the changed file (mypy reports 3 pre-existing, unrelated errors in cwlviewer.py that exist identically on main before this change).
• pytest tests/test_singularity.py tests/test_singularity_versions.py and the broader -k "singularity or docker" slice: all pass (remaining skips are pre-existing environment gates — no Docker/Singularity/Podman binary available — unrelated to this change).
• Confirmed import cwltool.singularity now succeeds with spython completely uninstalled, and cwltool --validate runs successfully end-to-end without spython installed.

Motivation

We ran into this while trying to depend on cwltool from an internal Apache-2.0-licensed project that only needs --validate-style validation. Being forced to pull in an MPL-2.0-licensed package for a feature we never use blocked our internal third-party license review. This is a minimal, behavior-preserving fix rather than a request to change cwltool's own dependency policy — happy to adjust the approach if maintainers prefer something different (e.g. a try/except ImportError guard instead of a bare deferred import).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.24%. Comparing base (9f6fcba) to head (99ddfb5).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2344   +/-   ##
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  Coverage   85.24%   85.24%           
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  Files          46       46           
  Lines        8622     8622           
  Branches     2020     2020           
=======================================
  Hits         7350     7350           
  Misses        806      806           
  Partials      466      466           

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