pymongo 4.0 support

[terencehonles]

This change follows up on top of #2600 and includes the following updates:

• merge master
• fixes for GEOHAYSTACK removal
• fixes for max_pool_size move
• fixes for map_reduce and inline_map_reduce removals
• fixes for eval removal
• fall back to the pythonLegacy UUID encoding for compatability with pymongo 3.x.
• fixes for SON.keys() not returning a list
• fixes for removed collection.update
• bubble up operation failure if an unsupported document count is attempted (The user can fetch the documents with len(list(queryset)) or modify the query as needed).
• fixes for db.authenticate removal

The test suite is now passing locally and I'm opening this up in the main repo to run against the CI. I had originally opened bagerard#20 to update #2600.

Fixes: #2598

[bagerard]

Wow, Thanks a lot for taking the lead on this @terencehonles. It looks great and the fact that you split it in logical commits made the review so much easier 👍

On top of the minor finding, may I ask you to:

• update the CHANGELOG and document the potential difference in behavior (and/or add reference to the pymongo migration page https://pymongo.readthedocs.io/en/stable/migrate-to-pymongo4.html). It's important to document any breaking change in both scenario: people upgrading just mongoengine, or upgrading both mongoengine+pymongo. If you don't feel like doing this, just let me know and I'll take over

• plus, feel free to add yourself to the AUTHORS file as well

[bagerard]

Alright, that looks great. Thanks again for all the work on this @terencehonles, much appreciated!