Declare SPDX-compatible BSD-3-Clause

[mrsimpson]

What this brings

This PR changes the declared license in the package's metadata.

What's the problem and what's the fix?

BSD is no SPDX-compatible identifier for licenses (see the list). It is a problem since automated tools will not be able to detect which license this package actually is under. Using BSD-3-Clause allows license checker tools to properly classify this package

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