venv lib_path detection compatible with 3.10+ (#41349)

diff --git a/tools/wpt/virtualenv.py b/tools/wpt/virtualenv.py
index c5e4503..742f572 100644
--- a/tools/wpt/virtualenv.py
+++ b/tools/wpt/virtualenv.py
@@ -81,7 +81,8 @@
             if IS_WIN:
                 site_packages = os.path.join(base, "Lib", "site-packages")
             else:
-                site_packages = os.path.join(base, "lib", f"python{sys.version[:3]}", "site-packages")
+                version = f"{sys.version_info.major}.{sys.version_info.minor}"
+                site_packages = os.path.join(base, "lib", f"python{version}", "site-packages")
 
         return site_packages