Merge tag 'v1.13.0rc2' into develop

Synapse 1.13.0rc2 (2020-05-14)
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Bugfixes
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- Fix a long-standing bug which could cause messages not to be sent over federation, when state events with state keys matching user IDs (such as custom user statuses) were received. ([\#7376](https://github.com/matrix-org/synapse/issues/7376))
- Restore compatibility with non-compliant clients during the user interactive authentication process, fixing a problem introduced in v1.13.0rc1. ([\#7483](https://github.com/matrix-org/synapse/issues/7483))

Internal Changes
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- Fix linting errors in new version of Flake8. ([\#7470](https://github.com/matrix-org/synapse/issues/7470))