Add classifier marking the project as typed to setup.cfg

Author: althonos

setup.cfg

@@ -30,6 +30,7 @@ classifiers =
     Programming Language :: Python :: Implementation :: CPython
     Programming Language :: Python :: Implementation :: PyPy
     Topic :: System :: Filesystems
+    Typing :: Typed
 project_urls =
     Bug Reports = https://github.com/PyFilesystem/pyfilesystem2/issues
     Documentation = https://pyfilesystem2.readthedocs.io/en/latest/