py/modmicropython: Add writable_ns(nsobj, bool) to make namespaces writable.
py/modmicropython: Add writable_ns(nsobj, bool) to make namespaces writable.

Calling writable_ns() on a type/module can make its attribute dictionary
writable/non-writable (depending on 2nd arg).

Change-Id: I65e714106966cd7aef41b3a518a4d495d249c3c4
Signed-off-by: Paul Sokolovsky <9dc5061f178bb11813e14f1ed02ba27eaf21fa5d@users.sourceforge.net>

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython