Migrate to SPDX license

I changed the operator between BSD and GPL because the text in license say:
  You may select, at your option, one of the above-listed licenses.
And some part of zstd.c are licensed under MIT

This is part of https://fedoraproject.org/wiki/Changes/SPDX_Licenses_Phase_2

python-zstandard.spec

@@ -21,7 +21,7 @@ Name: python-%{pypi_name}
 Version: 0.21.0
 Release: 3%{?dist}
 Summary: Zstandard bindings for Python
-License: BSD and GPLv2
+License: (BSD-3-Clause OR GPL-2.0-only) AND MIT
 URL: https://github.com/indygreg/python-zstandard
 Source0: %{pypi_source}