Change the PyAV version condition to depend on the platform

Author: ArtyomVancyan

setup.cfg

@@ -31,8 +31,8 @@ classifiers =
 packages =
     thumbnails
 install_requires =
-    av==8.0.0;python_version=='3.7'
-    av==9.2.0;python_version>'3.7'
+    av==8.0.0;sys_platform=='win32'
+    av==9.2.0;sys_platform!='win32'
     click>=8.0.3
     imageio-ffmpeg>=0.4.7
     imageio>=2.23.0