The pickle module was being imported from numpy.core.numeric. It was
defined there in order to use pickle5 when available in Python3 and
cpickle in Python2. The numpy.compat module seems a better place for
that.

All imports of pickle from numpy modules are now done this way:
>>> from numpy.core.numeric import pickle

Also, some loops on protocol numbers are added over pickle tests that
were not caught from #12090

That function is nose specific and has not worked since `__init__` files
were added to the tests directories.

This change _NoValue from a class to an instance, which is more inline with the builtin None.

Fixes gh-8991, closes gh-9592

Reloading currently causes problems because global classes defined in
numpy/__init__.py change their identity (a is b) after reload. The
solution taken here is to move those classes to a new non-reloadable
module numpy/_globals and import them into numpy from there.

Classes moved are ModuleDeprecationWarning, VisibleDeprecationWarning,
and _NoValue.

Closes #7844.

There seems to be little use in reloading numpy as any changed modules
that are imported into numpy would need to be reloaded first in order to
see any changes. Furthermore, reloading causes problems as global
classes defined in numpy/__init__.py change their identity. Hence we
raise a RuntimeError when an attempt to reload numpy is made.

Closes #7844.