inspect: Fix isgenerator logic.

Also optimise both `isgenerator()` and `isgeneratorfunction()` so they use
the same lambda, and don't have to create it each time they are called.

Fixes issue #997.

Signed-off-by: Damien George <damien@micropython.org>

python-stdlib/inspect/inspect.py

@@ -1,5 +1,7 @@
 import sys
 
+_g = lambda: (yield)
+
 
 def getmembers(obj, pred=None):
     res = []
@@ -16,11 +18,11 @@ def isfunction(obj):
 
 
 def isgeneratorfunction(obj):
-    return isinstance(obj, type(lambda: (yield)))
+    return isinstance(obj, type(_g))
 
 
 def isgenerator(obj):
-    return isinstance(obj, type(lambda: (yield)()))
+    return isinstance(obj, type((_g)()))
 
 
 class _Class: