bpo-1635741: Port _codecs extension module to multiphase initialization (PEP 489)

[shihai1991]

https://bugs.python.org/issue1635741

Automerge-Triggered-By: @encukou

[ncoghlan]

LGTM (given the relative simplicity of the module), but until I've reviewed a few more of these, I'd like to get a +1 from @encukou as well.

[encukou]

The catch it that the simple module exposes complex API, and multiphase init asserts that the module is expected to support sub-interpreters. Any module that manipulates some registry should get some extra attention :)
All looks OK here, since thePyCodec_* functions all use interpreter-specific state.
@Dormouse759, do you want to take a look?

[miss-islington]

@shihai1991: Status check is done, and it's a success ✅ .

[miss-islington]

Sorry, I can't merge this PR. Reason: Base branch was modified. Review and try the merge again..

[shihai1991]

Thanks, guys.

[encukou]

Thank you!