Don't allow slashes in Path Templating segments

According to OAI/OpenAPI-Specification#892
slashes aren't (yet?) supported in path templating parts, so change
the Path class to adhere to that.

This fixes a problem where declaring the paths
`/api/foo/{id}/` and `/api/foo/{id}/quux/` (in that order) would
cause the latter route to never be matched.

lepo/path.py

@@ -30,7 +30,7 @@ def _build_view_name(self):
     def _build_regex(self):
         return re.sub(
             PATH_PLACEHOLDER_REGEX,
-            lambda m: '(?P<%s>.+?)' % m.group(1),
+            lambda m: '(?P<%s>[^/]+?)' % m.group(1),
             self.path,
         ).lstrip('/') + '$'