go back to using the "or any later version" clause of GPLv3

I think arguments that "Such new versions will be similar in spirit to
the present version", in my own reading and readings such as in
https://www.draketo.de/software/gpl-or-later, convince me for now that
it is acceptable to allow the "or later" for compatibility and future
problem's sake

Signed-off-by: Brian S. Stephan <bss@incorporeal.org>

pyproject.toml

@@ -6,7 +6,7 @@ build-backend = "setuptools.build_meta"
 name = "incorporeal-cms"
 description = "A lightweight static site generator for Markdown-based sites."
 readme = "README.md"
-license = {text = "GPL-3.0-only"}
+license = {text = "GPL-3.0-or-later"}
 authors = [
 	{name = "Brian S. Stephan", email = "bss@incorporeal.org"},
 ]
@@ -16,7 +16,7 @@ dynamic = ["version"]
 classifiers = [
 	"Framework :: Flask",
 	"Programming Language :: Python :: 3",
-	"License :: OSI Approved :: GNU General Public License v3 (GPLv3)",
+	"License :: OSI Approved :: GNU General Public License v3 or later (GPLv3+)",
 	"Operating System :: OS Independent",
 	"Topic :: Text Processing :: Markup :: Markdown",
 ]