Replace relative imports by absolute ones in structure

[tobiasdiez]

Relative imports are not used consistently in the codebase and result in issues for doctesting with pytest (which admittedly is a limitation of pytest). We normalize the relative imports in sage.structure to be absolute imports.

📝 Checklist

• The title is concise, informative, and self-explanatory.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

[github-actions]

Documentation preview for this PR (built with commit 460d967; changes) is ready! 🎉

[tornaria]

Awesome: this should fix the fact that cythonize out of tree code that needs to use cimports from sage is currently broken (cython 3.0.4):

$ cat test.pyx 
from sage.structure.element cimport *
$ cythonize test.pyx
$ cythonize test.pyx 
Compiling /tmp/test.pyx because it changed.
[1/1] Cythonizing /tmp/test.pyx
/usr/lib/python3.12/site-packages/Cython/Compiler/Main.py:381: FutureWarning: Cython directive 'language_level' not set, using '3str' for now (Py3). This has changed from earlier releases! File: /tmp/test.pyx
  tree = Parsing.p_module(s, pxd, full_module_name)

Error compiling Cython file:
------------------------------------------------------------
...
from .sage_object cimport SageObject
^
------------------------------------------------------------

/usr/lib/python3.12/site-packages/sage/structure/element.pxd:1:0: 'sage/structure/sage_object.pxd' not found

Error compiling Cython file:
------------------------------------------------------------
...
from .sage_object cimport SageObject
^
------------------------------------------------------------

/usr/lib/python3.12/site-packages/sage/structure/element.pxd:1:0: 'sage/structure/sage_object/SageObject.pxd' not found

Error compiling Cython file:
------------------------------------------------------------
...
from .sage_object cimport SageObject
from .parent cimport Parent
^
------------------------------------------------------------

/usr/lib/python3.12/site-packages/sage/structure/element.pxd:2:0: 'sage/structure/parent.pxd' not found

Error compiling Cython file:
------------------------------------------------------------
...
from .sage_object cimport SageObject
from .parent cimport Parent
^
------------------------------------------------------------

/usr/lib/python3.12/site-packages/sage/structure/element.pxd:2:0: 'sage/structure/parent/Parent.pxd' not found

Error compiling Cython file:
------------------------------------------------------------
...

cdef unary_op_exception(op, x)
cdef bin_op_exception(op, x, y)


cdef class Element(SageObject):
                   ^
------------------------------------------------------------

/usr/lib/python3.12/site-packages/sage/structure/element.pxd:150:19: First base of 'Element' is not an extension type

Error compiling Cython file:
------------------------------------------------------------
...
cdef unary_op_exception(op, x)
cdef bin_op_exception(op, x, y)


cdef class Element(SageObject):
    cdef Parent _parent
         ^
------------------------------------------------------------

/usr/lib/python3.12/site-packages/sage/structure/element.pxd:151:9: 'Parent' is not a type identifier
Traceback (most recent call last):
  File "/usr/bin/cythonize", line 33, in <module>
    sys.exit(load_entry_point('Cython==3.0.4', 'console_scripts', 'cythonize')())
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/site-packages/Cython/Build/Cythonize.py", line 251, in main
    _cython_compile_files(all_paths, options)
  File "/usr/lib/python3.12/site-packages/Cython/Build/Cythonize.py", line 67, in _cython_compile_files
    ext_modules = cythonize(
                  ^^^^^^^^^^
  File "/usr/lib/python3.12/site-packages/Cython/Build/Dependencies.py", line 1154, in cythonize
    cythonize_one(*args)
  File "/usr/lib/python3.12/site-packages/Cython/Build/Dependencies.py", line 1321, in cythonize_one
    raise CompileError(None, pyx_file)
Cython.Compiler.Errors.CompileError: /tmp/test.pyx

I think changing the relative imports to absolute ones makes this work.

[tornaria]

LGTM, the issue I mentioned is so annoying that I'm actually rebuilt my system sagemath with this. All test long passed. It fixes the issue of out of tree compilation, so I'll raise the priority.

[tobiasdiez]

Happy that this even resolved some other issues. Thanks for the quick review. Matthias and I have opened a few other PRs doing similar replacements.