cysignals: interrupt and signal handling for Cython

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Cython and interrupts

When writing Cython <http://cython.org/>_ code, special care must be
taken to ensure that the code can be interrupted with CTRL-C.
Since Cython optimizes for speed, Cython normally does not check for
interrupts. For example, code like the following cannot be interrupted
in Cython::

while True:
    pass

The cysignals package provides mechanisms to handle interrupts (and other
signals and errors) in Cython code.

Requirements

• Python >= 3.6
• Cython >= 0.28
• Sphinx >= 1.6 (for building the documentation)

Links

• cysignals on the Python package index: https://pypi.org/project/cysignals/
• cysignals code repository and issue tracker on GitHub: https://github.com/sagemath/cysignals
• full cysignals documentation on Read the Docs: http://cysignals.readthedocs.io/

Changelog

1.12.0 (release candidate)
^^^^^^^^^^^^^^^^^^^^^^^^^^

• Remove optional compile-time dependency on PARI/GP. [#166]

1.11.4 (2023-10-07)
^^^^^^^^^^^^^^^^^^^

• Include generated configure script in the sdist again.

1.11.3 (2023-10-04)
^^^^^^^^^^^^^^^^^^^

• Add support for Cython 3. [#174, #176, #182, #187]
• Add support for Python 3.12.
• Replace fprintf by calls to write, which is async-signal-safe according to POSIX. [#162]
• Introduce a general hook to interface with custom signal handling. [#181]

1.11.2 (2021-12-15)
^^^^^^^^^^^^^^^^^^^

• Drop assembly code added after 1.10.3 that is not portable.

1.11.0 (2021-11-26)
^^^^^^^^^^^^^^^^^^^

• Drop Python 2 support; bump minimum Python version to 3.6. [#142]
• Fixed compilation with glib 3.34. [#151]
• Improved testing. [#139, #152, #154]

1.10.3 (2021-03-16)
^^^^^^^^^^^^^^^^^^^

• Improved installation of cysignals with pip install -e. [#130]

• Fixed compilation of OpenMP modules that also use cysignals. [#128]

• Fixed segmentation fault that could occur when sig_occurred() is
  called recursively during garbage collection. [#127]

• Improved error reporting of signals that occurred inside sig_on() as
  opposed to outside them.

• Fixed bug in the cysignals_example package. [#113]

For changes in previous releases, see the best source available is to
compare git tags: https://github.com/sagemath/cysignals/tags