update to spdx license key, version bump

Author: nschloe

setup.cfg

@@ -1,13 +1,13 @@
 [metadata]
 name = pygalmesh
-version = 0.6.3
+version = 0.6.4
 url = https://github.com/nschloe/pygalmesh
 author = Nico Schlömer
 email = nico.schloemer@gmail.com
 description = Python frontend to CGAL's 3D mesh generation capabilities
 long_description = file: README.md
 long_description_content_type = text/markdown
-license = GPLv3
+license = GPL-3.0-or-later
 license_files = LICENSE
 classifiers =
     Development Status :: 4 - Beta