fix: allow omitting the strip_prefix in tool_versions (#975)

Author: lummax

python/repositories.bzl

@@ -352,7 +352,6 @@ python_repository = repository_rule(
         ),
         "strip_prefix": attr.string(
             doc = "A directory prefix to strip from the extracted files.",
-            mandatory = True,
         ),
         "url": attr.string(
             doc = "The URL of the interpreter to download",