Fix async Python functors invoking from multiple C++ threads (pybind#1587)

Ensure GIL is released after functor destructor finished (not only
during functor execution as in previous implementation).

Author: uentity

include/pybind11/functional.h

@@ -54,12 +54,15 @@ struct type_caster<std::function<Return(Args...)>> {
             }
         }
 
-        value = [func](Args... args) -> Return {
+        value = [func](Args... args) mutable -> Return {
             gil_scoped_acquire acq;
-            object retval(func(std::forward<Args>(args)...));
+            // move into local var to ensure GIL is released AFTER functor destructor is called
+            function f(std::move(func));
+            object retval(f(std::forward<Args>(args)...));
             /* Visual studio 2015 parser issue: need parentheses around this expression */
             return (retval.template cast<Return>());
         };
+
         return true;
     }