Dark mode: add class to invert image brightness

[hugovk]

For python/cpython#103960.

Re: #44 (comment).

Add a class to invert image brightness (but not contrast), similar to how we handled it for the PEPs repo: python/peps#2409 and python/peps#2949.

We can then add this class to images like this:

.. figure:: hashlib-blake2-tree.png
   :alt: Explanation of tree mode parameters.
   :class: invert-in-dark-mode

Which changes this:

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Into:

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And light mode remains unchanged:

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This can be merged before or after python/cpython#103983.