Standardize 'license' field

[remram44]

The LICENSE.md file is exactly the BSD-3-Clause license, so you could probably set the field in setup.py to BSD-3-Clause (that's the official SPDX identifier for the "BSD 3-Clause "New" or "Revised" License").

I noticed that there used to be more text in LICENSE.md, however you removed that in #74 and you seemed to want GitHub to automatically report this repository as BSD? So hopefully you're probably ok with it being in the PyPI metadata as well.

If I'm wrong and "BSD-like" is what it's got to be, apologies, at least other people finding that field and wondering what the situation is can hopefully find this issue.

See also #72