[pkg] Drop unused EMPTY_FILE global variable

[tox] Do not install package in "develop" This might shadow so packaging issues. And user can still pass --develop to tox.

[pkg] Bump version to 3.23.2

pep8 bits

[schema] Add a method on yams constraints to compute its unique name used to identify it in the DB backend.

[migration] Test and fix case of addition of an attribute with some serialized constraint It was crashing on attempt to remove an unexisting constraint. This code may be removed safely as it's expected to be handled by the constraint removal hook. Notice that adding a vocabulary on shortpara makes yams change the varchar max size. This is arguable but not the point of this test, so simply adapt the max size defined in the schema coherently so we don't have to bother with that at this point.

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[schema2sql] Drop most of the DB DROP related code This code is dead for a while, it has been superseded by `sql_drop_all_user_tables`.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