Fix pythonfinder bug unnesting python versions

Signed-off-by: Dan Ryan <dan@danryan.co>

pipenv/vendor/pythonfinder/models/__init__.py

@@ -4,7 +4,7 @@ import abc
 import operator
 import six
 from itertools import chain
-from ..utils import KNOWN_EXTS
+from ..utils import KNOWN_EXTS, unnest
 
 
 @six.add_metaclass(abc.ABCMeta)
@@ -88,7 +88,7 @@ class BasePath(object):
             if self.is_python and version_matcher(self.as_python):
                 return self
             return
-        finder = ((child, child.as_python) for child in chain(*filter(None, self.pythons.values())) if child.as_python)
+        finder = ((child, child.as_python) for child in unnest(self.pythons.values()) if child.as_python)
         py_filter = filter(
             None, filter(lambda child: version_matcher(child[1]), finder)
         )

pipenv/vendor/pythonfinder/utils.py

@@ -8,6 +8,7 @@ import subprocess
 import sys
 from fnmatch import fnmatch
 from .exceptions import InvalidPythonVersion
+from itertools import chain
 
 try:
     from pathlib import Path
@@ -137,3 +138,9 @@ def fs_str(string):
 
 
 _fs_encoding = sys.getfilesystemencoding() or sys.getdefaultencoding()
+
+
+def unnest(item):
+    if isinstance(next((i for i in item), None), (list, tuple)):
+        return chain(*filter(None, item))
+    return chain(filter(None, item))