Fixes#4288.
- Default imports to THIRD_PARTY, so in effect we merge the FIRST_PARTY and THIRD_PARTY stubs. This means import order is no longer affected by whether typing_extensions is installed locally.
- Treat typing_extensions, _typeshed and some others as standard library modules.
Note that isort master is very different from the latest release; we'll have to do something
different if and when the next isort release comes out.
* Add empty stubs for gettext Translations subclasses
GNUTranslations and NullTranslations do not add any methods to the base
Translations class, so remove the TODO comment and just declare the
classes.
* Add missing types for py3 gettext.
