Python-arango

* Added new APIs for Pregel. Please refer to the [documentation](http://python-driver-for-arangodb.readthedocs.io/en/master/pregel.html) for more details.
* Updated documentation on thread-safety.

* Fixed a KeyError bug in `BaseCollection.properties` method.
* Added **pytest** to `test_requires` in setup.py.

* Added new parameter `deduplicate` to `Collection.add_hash_index` and `Collection.add_skiplist_index` methods.
* Added collection-specific user access management, and update database-specific user access management API. View more details [here](http://python-driver-for-arangodb.readthedocs.io/en/master/user.html).

* Added support for ArangoDB version **3.2**
* Added new parameters `smart`, `smart_field` and `shard_count` to `Database.create_graph` method.
* Added new parameter `drop_collections` to `Database.delete_graph` method.

* Duplicated several methods in `arango.client.ArangoClient` to `arango.database.Database` to allow non-root users access to the API operations (only root user could call them before). More information on the duplicated methods and their usage can be found in the following pages:

* http://python-driver-for-arangodb.readthedocs.io/en/master/wal.html
* http://python-driver-for-arangodb.readthedocs.io/en/master/admin.html
* http://python-driver-for-arangodb.readthedocs.io/en/master/user.html
* http://python-driver-for-arangodb.readthedocs.io/en/master/async.html

* Added new method `arango.client.ArangoClient.asynchronous` to replace the soon-to-be-deprecated method `arango.client.ArangoClient.async` (due to name conflict with Python 3.7+ keyword `async`). The changes are reflected in http://python-driver-for-arangodb.readthedocs.io/en/master/async.html.

* Improved the performance for large batch commits.
* Updated method `Collection.import_bulk` with new parameters.
* Added support for ArangoDB version **3.1.21**.