Non binary content schema

[rtibbles]

Summary

Changes the way we store our SQLAlchemy content schema to python modules, rather than binary files containing the pickles of the Python objects.
Upgrades SQLAlchemy (which we previously couldn't do beyond patch versions because the pickled Python objects depended on the internal APIs not changing).

Reviewer guidance

Does channel import work?

We should hold off merging this until #6873 is merged, so that this can be rebased, and the generation of a new content schema properly tested.

This should hopefully also fix #7004 due to the SQLAlchemy version upgrade.

---

Contributor Checklist

PR process:

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Testing:

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

Reviewer Checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[codecov]

Codecov Report

Merging #6994 into release-v0.14.x will increase coverage by 1.22%.
The diff coverage is 98.80%.

Impacted Files	Coverage Δ
...ore/content/management/commands/generate_schema.py	0.00% <0.00%> (ø)
kolibri/core/content/utils/sqlalchemybridge.py	84.40% <89.47%> (-0.34%)	⬇️
...content/contentschema/versions/content_schema_1.py	100.00% <100.00%> (ø)
...content/contentschema/versions/content_schema_2.py	100.00% <100.00%> (ø)
...content/contentschema/versions/content_schema_3.py	100.00% <100.00%> (ø)
...content/contentschema/versions/content_schema_4.py	100.00% <100.00%> (ø)
...t/contentschema/versions/content_schema_current.py	100.00% <100.00%> (ø)
...ntentschema/versions/content_schema_unversioned.py	100.00% <100.00%> (ø)
...contentschema/versions/content_schema_v020beta1.py	100.00% <100.00%> (ø)
...contentschema/versions/content_schema_v040beta3.py	100.00% <100.00%> (ø)
... and 12 more

[indirectlylit]

Is this something we want to target to 0.14?

[rtibbles]

The only reason we would need to is if we wanted to update SQLAlchemy in this release. We can leave this on develop though.

[rtibbles]

OK, looks like for Python 3.8 support we would need to merge this PR, so I think we should prioritize it.

[indirectlylit]

tested import and it works fine