refactor: drop sip_tpv, rely on astropy for TPV WCS (closes #713)

[cailmdaley]

Summary

Removes the sip_tpv dependency from shapepipe entirely. Modern astropy (≥5) parses TPV distortion natively via WCSLIB, as do SExtractor, PSFEx, and every other downstream WCS consumer in the pipeline, so the keyword-level pv_to_sip conversion that split_exp used to run was no longer buying us anything.

Dropping the conversion lets us drop sip_tpv itself — an unmaintained 2017 package whose v1.1 imports pkg_resources at module load and is therefore incompatible with setuptools>=81. That's the actual root cause of #713 (rewritten to reflect this). PR #714's setuptools<81 pin is a symptom workaround that becomes unnecessary once sip_tpv is gone.

What changed

Code

• split_exp_package/split_exp.py: drop import sip_tpv as stp and the stp.pv_to_sip(h) call; remove the now-dead transf_coord parameter from process/create_hdus; fix the module docstring.
• split_exp_runner.py, python_example_runner.py: remove sip_tpv from depends=[…].
• find_exposures_runner.py: remove sip_tpv from depends=[…] — spurious declaration, find_exposures_package never imported it.

Environment

• environment.yml, environment-dev.yml, Dockerfile, install_shapepipe: drop sip_tpv.
• docs/source/dependencies.md: drop the sip_tpv row.
• docs/source/refs.bib: drop the orphan shupe:12 entry.

Tests (new)

• src/shapepipe/tests/test_split_exp.py: three unittests that build a synthetic multi-CCD FITS with TPV distortion, run SplitExposures, and verify:
  1. Per-CCD output headers' WCS matches the source exposure's WCS at several pixel samples.
  2. The headers*.npy side-output round-trips pixel → world → pixel.
  3. flag output files are stored as int16.

All four modules in src/shapepipe/tests/ still pass locally (7/7).

Not touched

• scripts/jupyter/wcs.ipynb still imports sip_tpv — it's an exploratory notebook last edited ~2.5 years ago. Left alone to keep the diff scoped.

Interaction with open PRs

• fix: small develop bugs (#709, #711, #712, #713) #714 (develop-bugs) currently pins setuptools<81 as a workaround. Once this PR merges, that pin can come out. If fix: small develop bugs (#709, #711, #712, #713) #714 merges first the pin becomes dead code; happy to follow up with a cleanup commit in either direction.
• test: Tier 0-2 scaffolding (shell syntax, imports, entrypoints, runner metadata) #708 (testing scaffold) introduces tests/unit/ and includes an xfailed import test for shapepipe.modules.split_exp_package.split_exp. That test should xpass once either this PR or the setuptools<81 pin lands.

Reviewer Checklist

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