Move from flask-babelex to flask-babel

[demetrius-mp]

Since flask-babelex is discontinued, it would be great to add support for flask-babel instead of babelex in flask admin.

[gunlinux]

Started to work with migration, got some issue with testing and app context

[LeXofLeviafan]

Related items:

• flask-babelex out-of-date #2077
• Added flask-babel as fallback for flask-babelex #2270
• migrate flask-babelex to Flask-Babel #2288

Additionally, there's currently a bug which prevents flask-admin (the latest PyPI package) from working with flask-babelex when flask-babel is installed: trying to open an entry edit form results in AttributeError: 'Babel' object has no attribute 'locale_selector' (wtforms imports flask_wtf that uses flask_babel, which clearly picks up the instance produced by flask-babelex and fails to use it because it relies on a decorator to specify the locale-selector instead of a keyword argument). And yes, it works when flask-babel is not installed due to how flask-wtf handles it.

…When's the next release anyway? The latest version on PyPI is 1.6.0 which is over a year old by now, and it doesn't include flask-babel fallback support (since it was added merely half a year ago).

[samuelhwilliams]

This has been done by #2447 now - will be released in v2.0.0 🙏