Disallow k=None for the eye function

[BvB93]

Xref #143 and numpy/numpy#19969.

The spec currently claims that the k parameter of eye should accept either None or an integer, however not a single mention is made of None in the actual parameter-description. In fact, support for k=None seems to be absent from the limes of np.eye as well.

Based on this it seems the inclusion of the k: Optional[int] = ... annotation may have been accidental; hence the Optional encapsulation is removed in this PR. If this was a deliberate choice I'd instead propose to update the docs with a more explicit mention of None and its expected behavior.

[asmeurer]

This looks good to me, but others should confirm that this is indeed what was intended.

[rgommers]

Thanks @BvB93. This seems like a simple copy-paste mistake, or assuming "optional" means "don't have to specify" rather than "| None". This issue wasn't discussed in the original PR (gh-31). In it goes.