Flash message with Markup fails

[jeroen-80]

Upgrading my platform from Python 3.8 + Flask 3.0.3 + Flask-Session 0.8.0 with redis backend, to Python 3.11 + Flask 3.1.0 + Flask-Session 0.8.0 with redis backend. Same user code.

Issue: fancy flash message breaks on the new platform (work fine on the old platform).

Flash message:
flash(Markup('press the play button <i class="bi-play-btn-fill black"></i> below'), 'info')

Error:

[2024-12-10 19:01:28,998] ERROR in base: Failed to serialize session data: Encoding objects of type Markup is unsupported
[2024-12-10 19:01:28,998] ERROR in app: Exception on / [POST]
Traceback (most recent call last):
  File "/var/www/lib/python3.11/site-packages/flask/app.py", line 1511, in wsgi_app
    response = self.full_dispatch_request()
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/var/www/lib/python3.11/site-packages/flask/app.py", line 920, in full_dispatch_request
    return self.finalize_request(rv)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/var/www/lib/python3.11/site-packages/flask/app.py", line 941, in finalize_request
    response = self.process_response(response)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/var/www/lib/python3.11/site-packages/flask/app.py", line 1322, in process_response
    self.session_interface.save_session(self, ctx.session, response)
  File "/var/www/lib/python3.11/site-packages/flask_session/base.py", line 305, in save_session
    self._upsert_session(app.permanent_session_lifetime, session, store_id)
  File "/var/www/lib/python3.11/site-packages/flask_session/redis/redis.py", line 78, in _upsert_session
    serialized_session_data = self.serializer.encode(session)
                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/var/www/lib/python3.11/site-packages/flask_session/base.py", line 132, in encode
    return self.encoder.encode(dict(session))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: Encoding objects of type Markup is unsupported
[2024-12-10 19:01:29,002] ERROR in base: Failed to serialize session data: Encoding objects of type Markup is unsupported
[2024-12-10 19:01:29,002] ERROR in app: Request finalizing failed with an error while handling an error
Traceback (most recent call last):
  File "/var/www/lib/python3.11/site-packages/flask/app.py", line 1511, in wsgi_app
    response = self.full_dispatch_request()
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/var/www/lib/python3.11/site-packages/flask/app.py", line 920, in full_dispatch_request
    return self.finalize_request(rv)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/var/www/lib/python3.11/site-packages/flask/app.py", line 941, in finalize_request
    response = self.process_response(response)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/var/www/lib/python3.11/site-packages/flask/app.py", line 1322, in process_response
    self.session_interface.save_session(self, ctx.session, response)
  File "/var/www/lib/python3.11/site-packages/flask_session/base.py", line 305, in save_session
    self._upsert_session(app.permanent_session_lifetime, session, store_id)
  File "/var/www/lib/python3.11/site-packages/flask_session/redis/redis.py", line 78, in _upsert_session
    serialized_session_data = self.serializer.encode(session)
                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/var/www/lib/python3.11/site-packages/flask_session/base.py", line 132, in encode
    return self.encoder.encode(dict(session))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: Encoding objects of type Markup is unsupported

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/var/www/lib/python3.11/site-packages/flask/app.py", line 941, in finalize_request
    response = self.process_response(response)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/var/www/lib/python3.11/site-packages/flask/app.py", line 1322, in process_response
    self.session_interface.save_session(self, ctx.session, response)
  File "/var/www/lib/python3.11/site-packages/flask_session/base.py", line 305, in save_session
    self._upsert_session(app.permanent_session_lifetime, session, store_id)
  File "/var/www/lib/python3.11/site-packages/flask_session/redis/redis.py", line 78, in _upsert_session
    serialized_session_data = self.serializer.encode(session)
                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/var/www/lib/python3.11/site-packages/flask_session/base.py", line 132, in encode
    return self.encoder.encode(dict(session))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: Encoding objects of type Markup is unsupported