Explicitly support Python 3.8

Author: gahjelle

pyproject.toml

@@ -18,6 +18,7 @@ classifiers = [
     "Operating System :: POSIX :: Linux",
     "Programming Language :: Python :: 3.6",
     "Programming Language :: Python :: 3.7",
+    "Programming Language :: Python :: 3.8",
     "Topic :: Education",
     "Topic :: Software Development :: Libraries :: Python Modules",
     "Topic :: System :: Monitoring",