More unicode whack-a-mole

It seems pytest's very comprehensive CI sniffed out a few other places with similar bugs.  Ideally we should find all the places where args are not stringy and solve it at the source, but who knows how many people are relying on the implicit string conversion.  See [here](https://github.com/pytest-dev/pytest/blob/master/src/_pytest/config/__init__.py#L160-L166) for one such problem area (args with a single py.path.local instance is converted here, but a list or tuple containing some are not).

src/_pytest/config/argparsing.py

@@ -2,6 +2,8 @@ import six
 import warnings
 import argparse
 
+import py
+
 FILE_OR_DIR = "file_or_dir"
 
 
@@ -70,7 +72,8 @@ class Parser(object):
 
         self.optparser = self._getparser()
         try_argcomplete(self.optparser)
-        return self.optparser.parse_args([str(x) for x in args], namespace=namespace)
+        args = [str(x) if isinstance(x, py.path.local) else x for x in args]
+        return self.optparser.parse_args(args, namespace=namespace)
 
     def _getparser(self):
         from _pytest._argcomplete import filescompleter
@@ -106,7 +109,7 @@ class Parser(object):
         the remaining arguments unknown at this point.
         """
         optparser = self._getparser()
-        args = [str(x) for x in args]
+        args = [str(x) if isinstance(x, py.path.local) else x for x in args]
         return optparser.parse_known_args(args, namespace=namespace)
 
     def addini(self, name, help, type=None, default=None):