Move Code_of_Conduct.rst to the root level. (GH-8209)

Follow up from https://github.com/python/peps/pull/712#issuecomment-403591108:

"people using the repo without using GitHub still ought to be reminded of the COC" - GvR

diff --git a/.github/CODE_OF_CONDUCT.rst b/CODE_OF_CONDUCT.rst
similarity index 100%
rename from .github/CODE_OF_CONDUCT.rst
rename to CODE_OF_CONDUCT.rst