Update supported python versions in classifiers (#450)

... according to what are actually tested. Also bump the enforced
minimum python version to make sure a tested version is used.

Author: kajinamit

setup.py

@@ -17,10 +17,12 @@
     Programming Language :: Python
     Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
     Programming Language :: Python :: 3.10
+    Programming Language :: Python :: 3.11
+    Programming Language :: Python :: 3.12
+    Programming Language :: Python :: 3.13
     Programming Language :: Python :: Implementation :: CPython
     Programming Language :: Python :: Implementation :: PyPy
     License :: OSI Approved :: BSD License
@@ -132,7 +134,7 @@ def run_tests(self):
     platforms=['any'],
     license='BSD',
     classifiers=classifiers,
-    python_requires=">=3.6",
+    python_requires=">=3.8",
     install_requires=reqs('default.txt'),
     setup_requires=setup_requires,
     tests_require=reqs('test.txt'),