Fix defusedxml requirement

Remove release candidate version from accepted range to avoid having later release candidates being installed.

Fixes #911

Proposed changes

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[ ] Release (new release request)
[ ] Bugfix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Code style update (PEP8, lint, formatting, renaming, etc)
[ ] Refactoring (no functional changes, no api changes)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] Build related changes (build process, tests runner, etc)
[x] Other (please describe): Remove release candidate version from accepted range

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n/a? I have added tests that prove my fix is effective or that my feature works
n/a? I have added documentation to https://github.com/python-social-auth/social-docs

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python-social-auth / social-core