Handle missing globals in Python Channel destructors
Handle missing globals in Python Channel destructors

If the last reference to a Python object is at module scope, when
its destructor is run before program termination it may find that
the globals it requires no longer exist. Destructors of objects
likely to be stored at module global scope need to check that
globals exist before attempting to use them, to avoid warnings
being printed by the Python interpreter.

See grpc#17004

vjpai/grpc,carl-mastrangelo/grpc,jboeuf/grpc,grpc/grpc,ctiller/grpc,nicolasnoble/grpc,firebase/grpc,sreecha/grpc,sreecha/grpc,jtattermusch/grpc,carl-mastrangelo/grpc,donnadionne/grpc,muxi/grpc,vjpai/grpc,jtattermusch/grpc,firebase/grpc,ctiller/grpc,ejona86/grpc,grpc/grpc,vjpai/grpc,ejona86/grpc,grpc/grpc,jtattermusch/grpc,mehrdada/grpc,donnadionne/grpc,nicolasnoble/grpc,donnadionne/grpc,donnadionne/grpc,muxi/grpc,muxi/grpc,donnadionne/grpc,ejona86/grpc,jboeuf/grpc,ejona86/grpc,vjpai/grpc,jboeuf/grpc,jboeuf/grpc,muxi/grpc,ctiller/grpc,stanley-cheung/grpc,stanley-cheung/grpc,donnadionne/grpc,jtattermusch/grpc,grpc/grpc,ejona86/grpc,ejona86/grpc,stanley-cheung/grpc,grpc/grpc,muxi/grpc,mehrdada/grpc,nicolasnoble/grpc,ejona86/grpc,firebase/grpc,firebase/grpc,jtattermusch/grpc,jtattermusch/grpc,jboeuf/grpc,muxi/grpc,carl-mastrangelo/grpc,vjpai/grpc,ejona86/grpc,pszemus/grpc,mehrdada/grpc,grpc/grpc,sreecha/grpc,carl-mastrangelo/grpc,ejona86/grpc,stanley-cheung/grpc,carl-mastrangelo/grpc,mehrdada/grpc,firebase/grpc,ctiller/grpc,vjpai/grpc,jtattermusch/grpc,grpc/grpc,pszemus/grpc,carl-mastrangelo/grpc,donnadionne/grpc,nicolasnoble/grpc,sreecha/grpc,pszemus/grpc,ctiller/grpc,grpc/grpc,donnadionne/grpc,nicolasnoble/grpc,jtattermusch/grpc,pszemus/grpc,stanley-cheung/grpc,jtattermusch/grpc,carl-mastrangelo/grpc,muxi/grpc,pszemus/grpc,stanley-cheung/grpc,firebase/grpc,mehrdada/grpc,muxi/grpc,pszemus/grpc,vjpai/grpc,donnadionne/grpc,pszemus/grpc,jboeuf/grpc,vjpai/grpc,vjpai/grpc,nicolasnoble/grpc,firebase/grpc,grpc/grpc,ctiller/grpc,jboeuf/grpc,sreecha/grpc,stanley-cheung/grpc,pszemus/grpc,pszemus/grpc,nicolasnoble/grpc,vjpai/grpc,pszemus/grpc,ejona86/grpc,vjpai/grpc,carl-mastrangelo/grpc,nicolasnoble/grpc,ctiller/grpc,sreecha/grpc,jtattermusch/grpc,grpc/grpc,carl-mastrangelo/grpc,nicolasnoble/grpc,pszemus/grpc,muxi/grpc,ejona86/grpc,firebase/grpc,donnadionne/grpc,ctiller/grpc,mehrdada/grpc,firebase/grpc,ctiller/grpc,ctiller/grpc,donnadionne/grpc,sreecha/grpc,stanley-cheung/grpc,sreecha/grpc,firebase/grpc,vjpai/grpc,jboeuf/grpc,jboeuf/grpc,carl-mastrangelo/grpc,muxi/grpc,sreecha/grpc,mehrdada/grpc,muxi/grpc,ctiller/grpc,mehrdada/grpc,carl-mastrangelo/grpc,mehrdada/grpc,stanley-cheung/grpc,sreecha/grpc,ctiller/grpc,grpc/grpc,nicolasnoble/grpc,jtattermusch/grpc,pszemus/grpc,jboeuf/grpc,sreecha/grpc,donnadionne/grpc,stanley-cheung/grpc,jboeuf/grpc,muxi/grpc,jboeuf/grpc,jtattermusch/grpc,ejona86/grpc,mehrdada/grpc,nicolasnoble/grpc,nicolasnoble/grpc,sreecha/grpc,firebase/grpc,grpc/grpc,mehrdada/grpc,stanley-cheung/grpc,firebase/grpc,stanley-cheung/grpc,mehrdada/grpc,carl-mastrangelo/grpc