Update project metadata to use SPDX license expression

[cdce8p]

flit-core 3.12 was released today with support for compound license expressions. https://flit.pypa.io/en/stable/history.html#version-3-12

Metadata diff

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+License-Expression: Apache-2.0 OR BSD-2-Clause
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-Classifier: License :: OSI Approved :: Apache Software License
-Classifier: License :: OSI Approved :: BSD License
 ...