some setup.cfg fixes

Author: nschloe

setup.cfg

@@ -11,8 +11,7 @@ project_urls =
 long_description = file: README.md
 long_description_content_type = text/markdown
 license = MIT
-license_files = LICENSE
-platforms = any
+license_file = LICENSE
 classifiers =
     Development Status :: 4 - Beta
     License :: OSI Approved :: MIT License
@@ -27,9 +26,8 @@ classifiers =
 
 [options]
 packages = find:
-# importlib_metadata can be removed when we support Python 3.8+ only
 install_requires =
-    importlib_metadata
+    importlib_metadata;python_version<"3.8"
     matplotlib >= 1.4.0
     numpy
     Pillow