Spec(...) does not specify its category

[nthiery]

Spec(...) does not specify its category:

sage: C = Spec(ZZ)
sage: C.category()
Category of sets
sage: isinstance(C, C.category().element_class)
False

Caught with #7921; please write patch on top of it to avoid conflicts.

Here is the complete TestSuite? report, which also complains about an_element, ...::

sage: TestSuite(Spec(ZZ)).run()
running ._test_an_element() . . . fail
Traceback (most recent call last):
...
AssertionError: self.an_element() is not in self
------------------------------------------------------------
running ._test_category() . . . fail
Traceback (most recent call last):
...
AssertionError: category of self improperly initialized
------------------------------------------------------------
running ._test_elements() . . . 
  Failure in _test_category:
  Traceback (most recent call last):
  ...
  AssertionError
  ------------------------------------------------------------
  running ._test_not_implemented_methods() . . . pass
  running ._test_pickling() . . . fail
  Traceback (most recent call last):
  ...
  RuntimeError: ...BUG in map, returned None Point on Spectrum of Integer Ring defined by the Principal ideal (2) of Integer Ring <type 'sage.categories.morphism.CallMorphism'> Call morphism:
  ------------------------------------------------------------
  The following tests failed: _test_category, _test_pickling
running ._test_not_implemented_methods() . . . pass
running ._test_pickling() . . . pass
running ._test_some_elements() . . . fail
Traceback (most recent call last):
...
AssertionError: the object Point on Spectrum of Integer Ring defined by the Principal ideal (2) of Integer Ring in self.some_elements() is not in self
------------------------------------------------------------
The following tests failed: _test_an_element, _test_category, _test_elements, _test_some_elements

[novoselt]

#11599 claims to fix this, but I am not entirely sure what would it take to fix this ticket - the reported category is now schemes and only 3 of the TestSuite tests fail.