Support passing Dict[unicode, Any] in as **kwargs (#1966)

Fixes #1954.

Author: rowillia

mypy/checkexpr.py

@@ -1657,8 +1657,21 @@ def is_valid_var_arg(self, typ: Type) -> bool:
 
     def is_valid_keyword_var_arg(self, typ: Type) -> bool:
         """Is a type valid as a **kwargs argument?"""
-        return is_subtype(typ, self.chk.named_generic_type(
-            'builtins.dict', [self.named_type('builtins.str'), AnyType()]))
+        if self.chk.options.python_version[0] >= 3:
+            return is_subtype(typ, self.chk.named_generic_type(
+                'builtins.dict', [self.named_type('builtins.str'),
+                                  AnyType()]))
+        else:
+            return (
+                is_subtype(typ, self.chk.named_generic_type(
+                    'builtins.dict',
+                    [self.named_type('builtins.str'),
+                     AnyType()]))
+                or
+                is_subtype(typ, self.chk.named_generic_type(
+                    'builtins.dict',
+                    [self.named_type('builtins.unicode'),
+                     AnyType()])))
 
     def has_non_method(self, typ: Type, member: str) -> bool:
         """Does type have a member variable / property with the given name?"""

test-data/unit/python2eval.test

@@ -128,6 +128,28 @@ print '>', ['a', b'b', u'c']
 [out]
 > [u'a', 'b', u'c']
 
+[case testUnicodeLiteralsKwargs_python2]
+from __future__ import unicode_literals
+def f(**kwargs):  # type: (...) -> None
+  pass
+params = {'a': 'b'}
+f(**params)
+[out]
+
+[case testUnicodeStringKwargs_python2]
+def f(**kwargs):  # type: (...) -> None
+  pass
+params = {u'a': 'b'}
+f(**params)
+[out]
+
+[case testStrKwargs_python2]
+def f(**kwargs):  # type: (...) -> None
+  pass
+params = {'a': 'b'}
+f(**params)
+[out]
+
 [case testFromFutureImportUnicodeLiterals2_python2]
 from __future__ import unicode_literals
 def f(x: str) -> None: pass