[Refactoring] Split the big aggregate pipeline function in a function…

… for each stage.

Introduce a patch method, for an easy replacement of pymongo by mongo…

…mock across several clients and servers.

Use package version as recommended by @vmalloc.

Bump version to 3.11.1

Merge pull request mongomock#433 from pcorpet/pymongo-37

Unleash pymongo version for testing

v3.10.0

v3.9.0

Release 3.8.0

This release contains the following features and bugfixes

* Fix cursor.__getitem__ behavior mongomock#309 [@touilleMan]
* Raise exception when $unwind reference is not prefixed with '$' mongomock#307 [@MetrodataTeam]
* Support $type in find() mongomock#305 [@MetrodataTeam]
* Fix database property behavior mongomock#304 [@pcorpet]
* Support aggregate $out mongomock#303 [@MetrodataTeam]
* Skip missing fields in $project when aggregating mongomock#302 [@MetrodataTeam]
* Refactor Cursor constructor mongomock#301 [@edengreen]
* Aggregate projection support field rename mongomock#300 [@MetrodataTeam]
* Enable BulkWriteResult from collection.bulk_write mongomock#299 [@edengreen]
* Support $arrayElemAt in aggregate $project mongomock#298 [@MetrodataTeam]
* Return the copy of the document, not the actual document mongomock#291 [@Bernie]
* Fix pymongo bulk_write behavior mongomock#289 [@afouchet]
* Enable unset positional awareness mongomock#286 [@Bernie]
* Improve group _id capability mongomock#280 [@adamhicks]
* Add support for Cursor.rewind mongomock#277 [@gmacon]
* Support aggregate over literal value mongomock#274 [@ymost]
* Fix for updating with {} mongomock#272 [@bugrimov]

Basic $project support. Various operators.

Several improvements to aggregations.