v0.4.1

Fixed

• Fix ComdabInternalError raised when generating migrations updating some
  model extra; MigrationGeneratorPort.alter_*_extra methods are now
  correctly called in these cases (fixes #1)
• Fix ComdabForeignKeyConstraint.columns_mapping not treated as an opaque
  value in comparisons (generated .left_only / .right_only paths)
• Fix ComdabType not exported in comdab.models