[compat] Simplify kwarg detection code

This enables nuitka to compile youtube-dl.

youtube_dl/compat.py

@@ -297,7 +297,9 @@ def compat_getpass(prompt, *args, **kwargs):
 
 # Old 2.6 and 2.7 releases require kwargs to be bytes
 try:
-    (lambda x: x)(**{'x': 0})
+    def _testfunc(x):
+        pass
+    _testfunc(**{'x': 0})
 except TypeError:
     def compat_kwargs(kwargs):
         return dict((bytes(k), v) for k, v in kwargs.items())