[test] Add a migration test for update of a static vocabulary constraint This is a constraint serialized to be checked by the DB backend, hence ensure the old constraint is dropped and the new one is created.

[schema2sql] Avoid "parsing" SQL statements for database initialization A big SQL string was generated, then splitted. This caused bug if some value in the schema (eg vocabulary, default) contained the separator (';'). To properly fix this, yield each individual statement instead of generating a string. Closes #14050899

[schema2sql] Drop most of the DB DROP related code This code is dead for a while, it has been superseded by `sql_drop_all_user_tables`. Related to #14050899

[hooks] Use already imported module alias

[schema2sql] Rename as_sql to constraint_value_as_sql which is easier to grasp independantly of the context.

[schema2sql] Drop unused indent argument on aschema2sql Related to #14050899

[schema2sql] More docstrings and minor API improvements Along the way, renamed local `column` variable to `attr` where we're NOT manipulating a DB column name but a schema attribute. Related to #14050899

[schema2sql] inline eschema_attrs method * not a public API * doesn't makes things easier to grasp -> useless Related to #14050899

[schema2sql] Give a rdef to check_constraint This simplifies the API. Related to #14050899

merge 3.20.16 in 3.22