Python Requirements Update

Fix the issue that was preventing us from upgrading pytest.  pytest does some manipulation of test packages that prevents `pkg_resources` from loading resources from them, but used to contain a workaround for the problem.  That workaround was [removed](https://github.com/pytest-dev/pytest/issues/5392) in 4.6.0 as a performance enhancement when pytest switched from `pkg_resources` to `importlib-metadata` for its own entrypoint handling.  This tripped up one of our test modules which defined classes that loaded templates from inside a test package.  Moving these resources to the parent package fixes the problem.

More and more, `pkg_resources` is being abandoned in favor of `importlib-metadata` and `importlib_resources` as they have a simpler design with much better performance.  However, `importlib_resources` doesn't support loading files from any directory which isn't itself a Python package (and doesn't allow direct use of paths including directories within the package).  Jinja2 chose a [different approach](https://github.com/pallets/jinja/pull/1082) that we may want to emulate in our resource handling.

Also fixed usage of a removed `pytest.raises()` parameter and a bug in our configuration of the `common/lib` tests that became a problem after the upgrade.

BOM-1124

etree.tostring returns bytes instead of string which when converted
back to XML, to be rendered as etree element, presents stray
characters.

PROD-1116

Direct assignment to the forward side of a many-to-many set is deprecated Fixing this issue.

Remove unused screenshots.

Don't try to typset mathjax if Hub has not loaded.

SE-1071 send user_logged_in signal from my_user_info

BB-1744: Remove flaky test related to MockS3Mixin [FOLLOW-UP]

We don't need this anymore.

The MockS3Mixin prevents the correct setup of the ModuleStoreTestCase 
and made this test fail. Since the fix for this wasn't trivial, this 
test was skipped on python 3, and now is removed.

A user's last logged in value previously wasn't updated when the user
logs in through the oauth2 flow from the ios mobile app. Here we must
send the user_logged_in signal manually. It's implemented in the
my_user_info mobile rest api endpoint because adding it to where the
oauth2 flow happens is to complex, and the mobile app hits this endpoint
after a successful login anyway.

In our tests there are times when Mathjax has been initialized but
Hub is still undefied.  In this case don't queue up typesetting.

Make a change to cause tests to run.

They were created to work with a service that no longer exists.

* Switch make upgrade to Python 3.5

* bok-choy fixes, constrain django-debug-toolbar

* Remove flaky bok-choy test

fix uploaded image aspect ratio in tinymce

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

[BB-1506] Use a callable as the default value for CourseEnrollment.mode

BOM-1111

PROD-1113: Fix retry_failed_photo_verifications.

Updating User.is_authenticated and User.is_anonymous as properties

Fix html editor for adding �

This patch would fix html editor
for adding � characters in
html attributes whenever cursor
is inside quotes. � characters
are added on save.

PROD-1076

Don't encode the filename.

We fixed this for when we sand_tarballs for local but not for when we
used django storages.  This should fix the filename when we get the file
from storages.

Only store bytes to s3 via boto2.

https://github.com/boto/boto/issues/2868

In python 3 contentfile can act like a file but return unicode strings.
Boto doesn't like that and it causes issues when encoding the bytes for
transfer to s3.