[AIRFLOW-4934] Bump Flask to resolve Werkzeug ProxyFix

[nritholtz]

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https://issues.apache.org/jira/browse/AIRFLOW-4934

Description

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A follow up fix for #5563. Upgrade Flask version pin to ensure Werkzeug>=0.15.
Fix deprecation warning for ProxyFix import.

Tests

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Consumption of Werkzeug client (similar to #3983)

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Code Quality

• Passes flake8

[ashb]

LGTM. Waiting on CI then I will pull this and #5563 in to the 1.10 release.