Allow trailing slash in URL

Current pypistats.org returns 404 for urls with a trailing slash (e.g. https://pypistats.org/packages/dep-checker/). Many sites (including PyPI and python.org) accept either form, redirecting to one or the other (PyPI redirects to the "with trailing slash" form).

For consistency and ease of use pypistats should accept both forms. With flask it's fairly easy to accomplish.