Fix #308, indicate that md5 is not used in a security context

tldextract/cache.py

@@ -38,8 +38,12 @@ def get_pkg_unique_identifier() -> str:
 
     tldextract_version = "tldextract-" + version
     python_env_name = os.path.basename(sys.prefix)
+    md5_kwargs = {}
+    # for python >= 3.9, indicate that md5 is not used in a security context
+    if sys.version_info >= (3, 9):
+        md5_kwargs = {"usedforsecurity": False}
     # just to handle the edge case of two identically named python environments
-    python_binary_path_short_hash = md5(sys.prefix.encode("utf-8")).hexdigest()[:6]
+    python_binary_path_short_hash = md5(sys.prefix.encode("utf-8"), **md5_kwargs).hexdigest()[:6]
     python_version = ".".join([str(v) for v in sys.version_info[:-1]])
     identifier_parts = [
         python_version,