The "implicit" constraint generated by a

The "implicit" constraint generated by a
type such as Boolean or Enum will not generate an
ALTER statement when run on SQlite, which does not
support ALTER for the purpose of adding/removing
constraints separate from the column def itself.
While SQLite supports adding a CHECK constraint
at the column level, SQLAlchemy would need modification
to support this.
A warning is emitted indicating this
constraint cannot be added in this scenario.
#98