Add OpenDirector.addheaders and PEP 526 refactoring (fixes #2163) (#2164)
Add OpenDirector.addheaders and PEP 526 refactoring (fixes #2163) (#2164)

* Add OpenDirector.addheaders and replace type comments with annotations

* Use ClassVar/eliminate `= ...` and add BaseHandler.handler_order (exists in Py2 versions)

euresti/typeshed,li-dan/typeshed