Update sqlalchemy stubs (#68)

* Update sqlalchemy stubs

The SQLAlchemy project now provides stubs and a mypy plugin.

* Add sqlalchemy mypy plugin

* Re-add dropbox sqlalchemy stubs

* Remove duplicate link

README.md

@@ -50,6 +50,7 @@ Collection of awesome Python types, stubs, plugins, and tools to work with them.
 - [pyspark-stubs](https://github.com/zero323/pyspark-stubs) - Stubs for [PySpark](https://spark.apache.org/docs/latest/api/python/index.html).
 - [pythonista-stubs](https://github.com/hbmartin/pythonista-stubs) - Stubs for [Pythonista](http://omz-software.com/pythonista/docs/ios/).
 - [sqlalchemy-stubs](https://github.com/dropbox/sqlalchemy-stubs) - Stubs for [SQLAlchemy](https://github.com/sqlalchemy/sqlalchemy).
+- [sqlalchemy2-stubs](https://docs.sqlalchemy.org/en/14/orm/extensions/mypy.html) - Official stubs and mypy plugin for [SQLAlchemy](https://www.sqlalchemy.org).
 - [torchtyping](https://github.com/patrick-kidger/torchtyping) - Enhanced type annotations for [pytorch](https://pytorch.org/).
 - [typeshed](https://github.com/python/typeshed) - Collection of library stubs, with static types.
 - [wsgitypes](https://github.com/shabbyrobe/wsgitypes) - Typing for WSGI application implementers. These are not stub files, they're interfaces you mark support for to help typecheck WSGI conformance.