[pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

setup.cfg

diff --git a/setup.cfg b/setup.cfg
index ab80bd6..3c401fc 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -16,6 +16,7 @@
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
     Programming Language :: Python :: Implementation :: CPython
     Programming Language :: Python :: Implementation :: PyPy