playhouse.reflection.generate_models does not reflect field constraints
We don't currently support inspecting constraints besides "NOT NULL" and an implied "UNIQUE".
Using peewee 3.13.1 with PostgreSQL 9.6.15 and psycopg2 2.8.2, I noticed that a model class generated with `generate_models()` had a column that did not reflect its specified `constraints`:

```python
from peewee import TextField, Check
from playhouse.reflection import generate_models

class TestModel(db.Model):
    a_field = TextField(constraints=[Check("a_field != ''")])

TestModel.create_table()
TestModel.a_field.constraints
#> 
models = generate_models(db.database)
models["testmodel"]._meta.columns["a_field"].constraints
#> None
```

Here is the table definition:

```
# \d testmodel
                             Table "public.testmodel"
 Column  |  Type   | Collation | Nullable |                Default
---------+---------+-----------+----------+---------------------------------------
 id      | integer |           | not null | nextval('testmodel_id_seq'::regclass)
 a_field | text    |           | not null |
Indexes:
    "testmodel_pkey" PRIMARY KEY, btree (id)
Check constraints:
    "testmodel_a_field_check" CHECK (a_field <> ''::text)
```
