Deprecate global imports of CombinatorialObject, CombinatorialClass, MapCombinatorialClass

[Matthias Köppe]

Part of #12913 (Meta-ticket: Deprecate CombinatorialClass in favor of the EnumeratedSet's categories)

[Matthias Köppe]

New commits:

​231fe03	src/sage/combinat/all.py: Deprecate the global imports of CombinatorialObject, CombinatorialClass, MapCombinatorialClass
​81fcc92	src/doc/en/developer/coding_basics.rst: Replace use of CombinatorialObject in doctest

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​d4810bf	src/sage/misc/persist.pyx: Import CombinatorialObject in doctest here
​ad521cf	src/sage/combinat/combinat.py: Add deprecation warning in doctest output
​1d3a09b	src/sage/combinat/combinat.py: Explicitly import CombinatorialClass in some doctests

[Matthias Köppe]

The deprecation via lazy imports unfortunately does not work very well:

sage -t --random-seed=332440843511251861710123815794934176969 src/sage/combinat/combinat.py
**********************************************************************
File "src/sage/combinat/combinat.py", line 1198, in sage.combinat.combinat.CombinatorialObject.__eq__
Failed example:
    class Bar(Element, CombinatorialObject):
        def __init__(self, l):
            CombinatorialObject.__init__(self, l)
Exception raised:
    Traceback (most recent call last):
      File "/Users/mkoeppe/s/sage/sage-rebasing/worktree-gcc11/local/var/lib/sage/venv-python3.9/lib/python3.9/site-packages/sage/doctest/forker.py", line 694, in _run
        self.compile_and_execute(example, compiler, test.globs)
      File "/Users/mkoeppe/s/sage/sage-rebasing/worktree-gcc11/local/var/lib/sage/venv-python3.9/lib/python3.9/site-packages/sage/doctest/forker.py", line 1088, in compile_and_execute
        exec(compiled, globs)
      File "<doctest sage.combinat.combinat.CombinatorialObject.__eq__[7]>", line 1, in <module>
        class Bar(Element, CombinatorialObject):
    TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases

[Travis Scrimshaw]

Hmm...that is a bit strange. Perhaps something is not being resolved correctly? I don't know from a quick look what is going on. However, +1 for doing this ticket.