Add dpnp.ndarray.__format__ method

[antonwolfy]

The PR implements a special python method __format__ for dpnp.ndarray.

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/2662/index.html

[coveralls]

coverage: 81.403% (+0.002%) from 81.401%
when pulling 74829f7 on add-format-method-impl
into aec0164 on master.