Allow to activate development mode with WSGI application
With #14354 the traceback are only shown in development mode. But when setup a testing environment using WSGI it is annoying to not have traceback on the client. Testers can not make proper reports. I propose to add a `TRYTOND_DEV` environment variable to enable the development mode. Also as #14354 was a security fix applied to all branches, I think this feature should be back-ported.
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