slash handling issue in template URI normalization

[zzzeek]

Summary

There is an inconsistency in how leading slashes are handled during URI normalization in template lookup, which can lead to incorrect path resolution.

Details will be updated when fix is provided.

[sqla-tester]

Mike Bayer has proposed a fix for this issue in the main branch:

Fix path traversal via double-slash URI prefix in TemplateLookup https://gerrit.sqlalchemy.org/c/sqlalchemy/mako/+/6653