Make Store.namespaces an empty generator

[aucampia]

90f6fe5 changed

class Store:
    def namespaces(self):
	""" """
	if False:
	    yield None

to

class Store:
    def namespaces(self):
	""" """

This resulted in Store.namespaces no longer being an emtpy generator
by default (see https://stackoverflow.com/q/13243766).

The reason it was removed is because it is unreachable code, however I
did not consider that it would make the function an empty generator.

The reason why if False: yield None made it an empty generator is
because the presence of the yield keyword informs CPython that a
function is a generator:
https://www.python.org/dev/peps/pep-0255/#why-a-new-keyword-for-yield-why-not-a-builtin-function-instead

There are several ways of making an empty generator:

• if False: yield
• return and yield
• yield from ()
• ... more

Both if False: yield and return; yield results in same bytecode
however, which is that of an empty function, and they are both equally
confusing I would say.

yield from () is somewhat clearer, but the bytecode of the funciton
looks very different from an empty function (see https://stackoverflow.com/a/61496399)

So the best option I see is to just put back what was there and add a
comment and a test to prevent the issue from re-occuring.

Fixes #1431

[aucampia]

I rebased this onto #1433 to make the test pass, not sure what is going on there exactly though but just ping me for any changes here.

[ashleysommer]

Thanks @aucampia