Test dependency NCClient depends on lxml, which has no binary packages for Python 3.15 beta yet

[moreati]

https://github.com/mitogen-hq/mitogen/actions/runs/27867074788/job/82472849955?pr=1536

Run set -o errexit -o nounset -o pipefail
py315-m_ans-ans14-s_lin: install_deps> python -m pip --no-python-version-warning --disable-pip-version-check install 'ansible~=14.0' -r /home/runner/work/mitogen/mitogen/tests/requirements.txt -r /home/runner/work/mitogen/mitogen/tests/ansible/requirements.txt
  DEPRECATION: Setting PIP_CONSTRAINT will not affect build constraints in the future, pip 26.2 will enforce this behaviour change. A possible replacement is to specify build constraints using --build-constraint or PIP_BUILD_CONSTRAINT. To disable this warning without any build constraints set --use-feature=build-constraint or PIP_USE_FEATURE="build-constraint".
  error: subprocess-exited-with-error
  
  × Getting requirements to build wheel did not run successfully.
  │ exit code: 1
  ╰─> [3 lines of output]
      Building lxml version 6.1.1.
      Building without Cython.
      Error: Please make sure the libxml2 and libxslt development packages are installed.
      [end of output]

Latest lxml release is https://github.com/lxml/lxml/releases/tag/lxml-7.0.0a3, also w/o Python 3.15 wheels.

[moreati]

lxml-7.0.0a3 is still the latest release

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lxml-7.0.0b1 includes Python 3.15 wheels

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https://github.com/lxml/lxml/releases/tag/lxml-6.1.3-1 has CPython 3.15 wheels