Document working with "sqlalchemy>0.2.0" in virtualenvs and the induced side-effects

[tgrandje]

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As discussed in #46266 there are (and will be more) usecases for sqlalchemy>0.2.0 usage. As far as I know, this will stay there untill Airflow 3 is released. Until then, (I think) there is a good point for documenting on how to deal with this, even if this is not intended behaviour: pandas for instance cannot work with Airflow as of yet.

The current PR :

• adds a general warning on using dependencies in a virtualenv which are not compatible with airflow (this subject was not documented yet I think)
• adds a note in the virtualenv tutorial listing the known side effects to using sqlalchemy>0.2.0.

[tgrandje]

This is a followup for #47914

[potiuk]

We are working to have airflow 2-10-test branch green again , so this one is likely to fail for now - #48404

[github-actions]

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