Use SPDX identifier

BSD is amiguous. So let's use the correct SPDX identifier.
https://spdx.org/licenses/

Author: Shortfinga

setup.py

@@ -23,7 +23,7 @@
     author_email=smmap.__contact__,
     url=smmap.__homepage__,
     platforms=["any"],
-    license="BSD",
+    license="BSD-3-Clause",
     packages=find_packages(),
     zip_safe=True,
     python_requires=">=3.7",