Mako: Path traversal via double-slash URI prefix in TemplateLookup

Mako is a template library written in Python. Prior to 1.3.11, TemplateLookup.get_template() is vulnerable to path traversal when a URI starts with // (e.g., //../../../secret.txt). The root cause is an inconsistency between two slash-stripping implementations. Any file readable by the process can be returned as rendered template content when an application passes untrusted input directly to TemplateLookup.get_template(). This vulnerability is fixed in 1.3.11.