CI: Add 3.13-nogil build

[effigies]

Closes #1322.

This initial job installs and tests:

• numpy (nightly wheel)
• scipy (nightly wheel)
• pillow (sdist; no cp313t wheel issue/PR to link)
• matplotlib (sdist, pending BLD: Enable building Python 3.13 wheels for nightlies  matplotlib/matplotlib#28293)
• pyzstd (sdist)
• pydicom (git)

We do not currently test:

• h5py (pending WHL: enable cp313t nightly builds h5py/h5py#2449)
• indexed_gzip

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.34%. Comparing base (08f5c44) to head (880e13e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1339      +/-   ##
==========================================
+ Coverage   95.32%   95.34%   +0.02%     
==========================================
  Files         207      207              
  Lines       29084    29084              
  Branches     4863     4863              
==========================================
+ Hits        27724    27730       +6     
+ Misses        927      923       -4     
+ Partials      433      431       -2     

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[effigies]

Looks like we're correctly pulling in the cp313t wheel:

Collecting numpy>=2.1.dev0
  Downloading https://pypi.anaconda.org/scientific-python-nightly-wheels/simple/numpy/2.1.0.dev0/numpy-2.1.0.dev0-cp313-cp313t-manylinux_2_17_x86_64.manylinux2014_x86_64.whl (19.2 MB)

But the GIL is being enabled:

<frozen importlib._bootstrap>:488: RuntimeWarning: The global interpreter lock (GIL) has been enabled to load module 'numpy._core._multiarray_umath', which has not declared that it can run safely without the GIL. To override this behavior and keep the GIL disabled (at your own risk), run with PYTHON_GIL=0 or -Xgil=0.

Setting PYTHON_GIL=0 for py313 tests.

[effigies]

@larsoner Does this all look reasonable to you? With numpy 2.0 released, I've switched the dev build to target 3.13+numpy 2.1.dev0.