py/strict_mode: Actually handle constant namespace slots.
py/strict_mode: Actually handle constant namespace slots.

During run-time, they cannot be overriden. At import-time, issue warnings
when trying to to turn non-const slot to const and overriding already
const slot (with either const or non-const).

Change-Id: Ia4b8f7956489fee5696fa88d5e82d8f5e7d2c303
Signed-off-by: Paul Sokolovsky <9dc5061f178bb11813e14f1ed02ba27eaf21fa5d@users.sourceforge.net>

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