Fixed subvertpy "Assertion `svn_uri_is_canonical(child_uri, ((void *)0))' failed"

There is an error in `list` function of subvertpy `Client` when `path`
parameter ends with trailing slash. This error also displays when running tests.

python: /build/buildd/subversion-1.8.8/subversion/libsvn_subr/dirent_uri.c:1499: uri_skip_ancestor: Assertion `svn_uri_is_canonical(child_uri, ((void *)0))' failed.

This change strips the trailing slash, fixing the error.

Testing Done: Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/6215/