Improved an error message in 'mkpath()'. Tightened up some logic in 'native_path()'. Added 'subst_vars()' and '_check_environ()'.
Improved an error message in 'mkpath()'.
Tightened up some logic in 'native_path()'.
Added 'subst_vars()' and '_check_environ()'.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator