bpo-42866: Add traverse func to _multibytecodec.MultibyteCodec

[vstinner]

Convert _multibytecodec.MultibyteCodec type to a GC type and adds a
traverse function.

https://bugs.python.org/issue42866

[vstinner]

Example:

import _codecs_jp
import gc
codec = _codecs_jp.getcodec('cp932')
print(gc.get_referents(codec))

Output:

[<class '_multibytecodec.MultibyteCodec'>]

[vstinner]

I updated the PR to track/untrack an instance in the GC.

Example:

import _codecs_jp
import gc
codec = _codecs_jp.getcodec('cp932')
# instance -> type
print(gc.get_referents(codec) == [type(codec)])
# type -> instance
print(any(ref is codec for ref in gc.get_referrers(type(codec))))

Output with the PR:

True
True