Make Image.__array__ take optional dtype argument

[t-vi]

This is required by the numpy protocol.

Fixes #5571

[t-vi]

To demystify what is needed by __array__, here are the two calls numpy does, one with dtype one without. (And in particular, you don't need to implement numpy.array 's signature.)

https://github.com/numpy/numpy/blob/45c57078fd599194b49148fb66808f03490ee423/numpy/core/src/multiarray/ctors.c#L2453-L2458

This has been very stable and the reason the bug shows up in Pillow 8.3 is that it implemented only the version without the dtype passed in when switching to providing __array__ in #5379.

I looked at numpy's ndarray __array__ for inspiration that providing a non-identical dtype casts/copies rather than failing.

[t-vi]

I'm not entirely sure what to do with the codecov. It seems implausible to me that the line it points out in the Image.__array__ isn't covered by tests and I don't know what to think about the idea of tests being covered by tests.

[radarhere]

I expect that was just because some CI jobs had an intermittent failure and were not returning their coverage results - everything looks fine with your latest commit.

[t-vi]

OK, I leave the it to you to take this forward, then. Thank you for your suggestions.