Update to pymongo 3.7

[lafrech]

https://api.mongodb.com/python/current/changelog.html

Deprecated pymongo.collection.Collection.count() and pymongo.cursor.Cursor.count(). These two methods use the count command and may or may not be accurate, depending on the options used and connected MongoDB topology. Use count_documents() instead.

This PR is based on #148.

It adds a count method to PyMongoDocument. (I named it count and not count_documents to match umongo's TxMongoDocument. I have no objection to naming it count_documents.)

I'd do the same for MotorAsyncIODocument except it requires motor>=2.0, which is not supported.

Since we already require pymongo 3.x, I figured bumping the requirement wouldn't be too much of an issue and would save us the pain of supporting both 3.7 and 3.6-. I could rework to only add the new count method if using 3.7 and only require 3.7 in the tests. But I'm not sure it is worth the trouble.

[coveralls]

Coverage decreased (-9.9%) to 85.776% when pulling 3a37472 on dev_update_to_pymongo_3_7 into db076f6 on master.

[coveralls]

Coverage increased (+0.008%) to 95.704% when pulling 47f05b8 on dev_update_to_pymongo_3_7 into 9bc2f02 on master.

[touilleMan]

It adds a count method to PyMongoDocument. (I named it count and not count_documents to match umongo's TxMongoDocument. I have no objection to naming it count_documents.)

I'd better keep count_documents if it's the name in the pymongo driver (I guess people are more interested in consistency between umongo and the driver they use than between drivers)
Another possibility would be to provide both name (count and count_documents), as you like.

Since we already require pymongo 3.x, I figured bumping the requirement wouldn't be too much of an issue and would save us the pain of supporting both 3.7 and 3.6

Agree, let's keep things simple ;-)

[touilleMan]

otherwise seems good to me 👍

[lafrech]

Alright.

I changed to count_documents. Note that my implementation differs from pymongo's in that the filter parameter is optional and defaults to {}. I can change that if you prefer to adhere more strictly. I don't understand why pymongo didn't make this argument optional. Maybe their good reason would apply here, too.