fix bugs for testing with Redshift (include a new feature; add columun with UNIQUE)

[shimizukawa]

1. FAILED: test_inspectdb; inspectdb should suppress output 'id = AutoField(primary_key=True)'
2. FAILED: test_alter_size
3. FAILED: test_alter_size_for_unique

[shimizukawa]

FAILED: test_alter_size

E django.db.utils.NotSupportedError: cannot decrease a VARCHAR column size with default value

inspection

#96 (comment)

to be fixed

1. adding new column with decreased size
2. migrate data from old to new column.
3. drop old column.
4. rename new column.

[shimizukawa]

FAILED: test_alter_size_for_unique

E django.db.utils.NotSupportedError: ALTER TABLE ADD COLUMN does not support columns defined with UNIQUE or PRIMARY KEY constraints

inspection

While sequentially checking the SQL issued by the test code, I encountered an error when adding a column with a UNIQUE constraint.

=# CREATE TABLE foo (id INTEGER);
=# ALTER TABLE "foo" ADD COLUMN "name" varchar(20) UNIQUE;
ERROR: ALTER TABLE ADD COLUMN does not support columns defined with UNIQUE or PRIMARY KEY constraints

Therefore, while the previously implemented "resizing columns with a UNIQUE constraint" is functioning, the test code that uses "adding a column with a UNIQUE constraint" encountered an error. I did not notice this issue during CI because the validation was performed using Postgres instead of Redshift.

The SQL execution log is summarized in the comment at the following link:
#96 (comment)

To be fixed

I have now implemented support for "adding a column with a UNIQUE constraint."