Comments on (named) constraints

Adds support for comments on named constraints, including `ForeignKeyConstraint`, `PrimaryKeyConstraint`, `CheckConstraint`, `UniqueConstraint`, solving the [Issue 5667](https://github.com/sqlalchemy/sqlalchemy/issues/5667).

Supports only PostgreSQL backend.

### Description

Following the example of [Issue 1546](https://github.com/sqlalchemy/sqlalchemy/issues/1546), supports comments on constraints. Specifically, enables comments on _named_ ones — as I get it, PostgreSQL prohibits comments on unnamed constraints.

Enables setting the comments for named constraints like this:
```
Table(
   'example', metadata,
   Column('id', Integer),
   Column('data', sa.String(30)),
   PrimaryKeyConstraint(
       "id", name="id_pk", comment="id_pk comment"
    ),
   CheckConstraint('id < 100', name="cc1", comment="Id value can't exceed 100"),
   UniqueConstraint(['data'], name="uc1", comment="Must have unique data field"),
)
```

Provides the DDL representation for constraint comments and routines to create and drop them. Class `.Inspector` reflects constraint comments via methods like `get_check_constraints` .
### Checklist
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This pull request is:

- [ ] A documentation / typographical error fix
- [ ] A short code fix
- [x] A new feature implementation
	- Solves the issue 5667.
	- The commit message includes `Fixes: 5667`.
	- Includes tests based on comment reflection.

**Have a nice day!**

Fixes: #5667
Closes: #7742
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/7742
Pull-request-sha: 42a5d3c3e9ccf9a9d5397fd007aeab0854f66130

Change-Id: Ia60f578595afdbd6089541c9a00e37997ef78ad3

1 files changed, 32 insertions, 0 deletions

diff --git a/test/sql/test_metadata.py b/test/sql/test_metadata.py
index 1a070fcf5..33b6e130f 100644
--- a/test/sql/test_metadata.py
+++ b/test/sql/test_metadata.py
@@ -237,6 +237,7 @@ class MetaDataTest(fixtures.TestBase, ComparesTables):
             deferrable="Z",
             initially="Q",
             link_to_name=True,
+            comment="foo",
         )
 
         fk1 = ForeignKey(c1, **kw)
@@ -259,6 +260,7 @@ class MetaDataTest(fixtures.TestBase, ComparesTables):
             name="name",
             initially=True,
             deferrable=True,
+            comment="foo",
             _create_rule=r,
         )
         c2 = c._copy()
@@ -266,6 +268,7 @@ class MetaDataTest(fixtures.TestBase, ComparesTables):
         eq_(str(c2.sqltext), "foo bar")
         eq_(c2.initially, True)
         eq_(c2.deferrable, True)
+        eq_(c2.comment, "foo")
         assert c2._create_rule is r
 
     def test_col_replace_w_constraint(self):
@@ -3577,6 +3580,35 @@ class ConstraintTest(fixtures.TestBase):
         for c in t3.constraints:
             assert c.table is t3
 
+    def test_ColumnCollectionConstraint_copy(self):
+        m = MetaData()
+
+        t = Table("tbl", m, Column("a", Integer), Column("b", Integer))
+        t2 = Table("t2", m, Column("a", Integer), Column("b", Integer))
+
+        kw = {
+            "comment": "baz",
+            "name": "ccc",
+            "initially": "foo",
+            "deferrable": "bar",
+        }
+
+        UniqueConstraint(t.c.a, **kw)
+        CheckConstraint(t.c.a > 5, **kw)
+        ForeignKeyConstraint([t.c.a], [t2.c.a], **kw)
+        PrimaryKeyConstraint(t.c.a, **kw)
+
+        m2 = MetaData()
+
+        t3 = t.to_metadata(m2)
+
+        eq_(len(t3.constraints), 4)
+
+        for c in t3.constraints:
+            assert c.table is t3
+            for k, v in kw.items():
+                eq_(getattr(c, k), v)
+
     def test_check_constraint_copy(self):
         m = MetaData()
         t = Table("tbl", m, Column("a", Integer), Column("b", Integer))