[tornado] Announcing Motor 2.0

[A. Jesse Jiryu Davis]

I've just released version 2.0 of Motor, my async driver for Python and
MongoDB. This version drops support for MongoDB 2.6 and adds MongoDB 4.0
features, including multi-document transactions, and change stream
notifications on entire databases or entire MongoDB servers. It adds
support for Python 3.7. This version of Motor requires PyMongo 3.7 or later.

To support multi-document transactions, Motor had to make breaking changes
to the session API and release a major version bump. Since this is a major
release it also deletes many helper methods and APIs that had been
deprecated over the time since Motor 1.0, most notably the old CRUD methods
insert, update, remove, and save, and the original callback-based API. Read
the Motor 2.0 Migration Guide
<http://motor.readthedocs.io/en/stable/migrate-to-motor-2.html> carefully
to upgrade your existing Motor application. Motor 2.0 also includes a
Python 3.7 compatibility fixes.

Motor 2.0's changes have a big impact for some applications, so I created a
bridge release that raises DeprecationWarning for all the APIs that are
deleted in Motor 2.0, especially the callback API, which hasn't been
deprecated before. Those DeprecationWarnings plus the Python 3.7
compatibility fixes constitute Motor 1.3.1.

For applications that only need Python 3.7 compatibility, I've created
Motor 1.2.5, which includes the Python 3.7 fixes and no other changes.

Please read the full changelog for Motor 2.0
<http://motor.readthedocs.io/en/latest/changelog.html>, try it out, and let
me know how the new driver works for you.

Peace,
A. Jesse Jiryu Davis