Fix Python 3.7 and PEP 479 incompatibility

With [PEP 479](https://www.python.org/dev/peps/pep-0479/) implemented in Python 3.7, raised `StopIteration` is now translated into `RuntimeError`, which breaks `mkdocs build` command.

The recommended way to exit generators is to use `return` instead of raising `StopIteartion`: https://www.python.org/dev/peps/pep-0479/#id39

Author: Konstantin Molchanov

mkdocs/nav.py

@@ -334,7 +334,7 @@ def _follow(config_line, url_context, config, header=None, title=None):
             header.children.append(page)
 
         yield page
-        raise StopIteration
+        return
 
     elif not isinstance(config_line, dict):
         msg = ("Line in 'page' config is of type {0}, dict or string "
@@ -347,15 +347,15 @@ def _follow(config_line, url_context, config, header=None, title=None):
             "config contains an invalid entry: {0}".format(config_line))
     elif len(config_line) == 0:
         log.warning("Ignoring empty line in the pages config.")
-        raise StopIteration
+        return
 
     next_cat_or_title, subpages_or_path = next(iter(config_line.items()))
 
     if isinstance(subpages_or_path, utils.string_types):
         path = subpages_or_path
         for sub in _follow(path, url_context, config, header=header, title=next_cat_or_title):
             yield sub
-        raise StopIteration
+        return
 
     elif not isinstance(subpages_or_path, list):
         msg = ("Line in 'page' config is of type {0}, list or string "