BB-1744: Bump pymongo version and fix course export issue

[giovannicimolin]

This commit upgrades the version of pymongo from 2.x to 3.x, removing usages to deprecated functions usage and fixing tests where necessary.

This version of pymongo supports MongoDB 2.x all the way up to 4.2, and this ensures that the platform will be able to run on a supported MongoDB version in the next release.

(cherry picked from commit 9b69d5a62684dc3d9d86b7e71081bebd432bd7d9)

See discussion and testing instructions on upstream ticket https://github.com/edx/edx-platform/pull/21197.

Sandboxes:
Mongo 3.2: https://console.opencraft.com/instance/17030/
Mongo 3.6: https://console.opencraft.com/instance/17070/
Mongo 4.0: https://console.opencraft.com/instance/17080/

Reviewers:

• @viadanna or @lgp171188

[lgp171188]

@giovannicimolin, the changes look good. I have left a few comments/questions. Can you check those out? I know this PR only has the backported changes from the upstream PR, but I left comments on the actual changes since I didn't review that,

[lgp171188]

@giovannicimolin, while testing this on the sandbox set up for MongoDB 3.6, I found some exceptions in the celery worker logs during the import, which may not be directly related to the changes made in this PR.

Traceback (most recent call last):
  File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/celery/worker/consumer.py", line 465, in on_task_received
    strategies[type_](message, body,
KeyError: u'openedx.core.djangoapps.bookmarks.tasks.update_xblock_cache'

and

Traceback (most recent call last):
  File "/edx/app/edxapp/edx-platform/common/lib/xmodule/xmodule/contentstore/content.py", line 440, in generate_thumbnail
    with Image.open(source) as image:
  File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/PIL/Image.py", line 2687, in open
    % (filename if filename else fp))
IOError: cannot identify image file <StringIO.StringIO instance at 0x7f7d0a8f6830>

FYI.

[giovannicimolin]

@lgp171188 I've debugged the issues:
I didn't dig to deep on the first one, but it's unrelated to these changes.

I've managed to reproduce the issue on the generate_thumbnail function: the assets that fail are corrupt images from the demonstration course (maybe this is intentional to test the import reliability?).

[lgp171188]

@giovannicimolin, I have resolved all my questions/comments after checking your responses. I also ran the python tests locally in my devstack and the few errors/failures that I saw were not related to the changes made in this PR. 👍

[lgp171188]

👍

• I tested this in the sandboxes and didn't find any issues. I also ran the tests locally and didn't see any errors or failures related to the changes in this PR.
• I read through the code
• I checked for accessibility issues NA
• Includes documentation NA