Use SPDX short code for license in Pyproject.toml

This will ensure that downstream tools e.g. Pypi are able to correctly map the license to a known one instead of marking it as Proprietary.

pyproject.toml

@@ -4,7 +4,7 @@ version = "1.11.6"  # Remember to also change __init__.py version
 description = 'A Django authentication backend for Microsoft ADFS and AzureAD'
 authors = ['Joris Beckers <joris.beckers@gmail.com>']
 maintainers = ['Jonas Krüger Svensson <jonas-ks@hotmail.com>', 'Sondre Lillebø Gundersen <sondrelg@live.no>']
-license = 'BSD'
+license = 'BSD-1-Clause'
 homepage = 'https://github.com/snok/django-auth-adfs'
 repository = 'https://github.com/snok/django-auth-adfs'
 documentation = 'https://django-auth-adfs.readthedocs.io/en/latest'