pytest and relative imports

[mkoeppe]

(from #33531 comment:14):

Out of all the files *_test.py in sage source tree (9 files), the only one that fails to pytest is sage/structure/sage_object_test.py which gives me the following error:

=================================================================================== ERRORS ===================================================================================
____________________________________________________________ ERROR collecting sage/structure/sage_object_test.py _____________________________________________________________
ImportError while importing test module '/home/tornaria/src/sage/sage-git/src/sage/structure/sage_object_test.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
src/sage/structure/sage_object_test.py:3: in <module>
    from .sage_object import SageObject
E   ImportError: attempted relative import with no known parent package
========================================================================== short test summary info ===========================================================================
This goes away if I change the import to be absolute (sage.structure.sage_object)

CC: @tornaria @tobiasdiez

Component: refactoring

Issue created by migration from https://trac.sagemath.org/ticket/33783