Accept deferrable kwarg for schema editor SQL (#86)

This follows Django 3.1.x's lead in adding `deferrable` as a keyword
argument to the `_create_unique_sql` function, following their
implementation in the schema editor object.  Without accepting this
keyword arg, migrations will crash like so:

```
...
  File "lib/python3.8/site-packages/django/db/backends/base/schema.py", line 360, in add_constraint
    sql = constraint.create_sql(model, self)
  File "lib/python3.8/site-packages/django/db/models/constraints.py", line 118, in create_sql
    return schema_editor._create_unique_sql(
TypeError: _create_unique_sql() got an unexpected keyword argument 'deferrable'
```

This also adjusts the implementation to call
`self_deferrable_constraint_sql(...)` with the deferrable arg; in this
backend's case the return value is an empty string (like it is currently
hardcoded).  Essentially it's the same result but allows flexibility if
this backend ever supported deferrable constraints.

Backwards compatibility is maintained by checking for the passing of a
deferrable keyword argument to the create sql function (not provided on
earlier Django versions before 3.1) and that the database backend
supports deferral for unique indexes.

Author: davidjb

sql_server/pyodbc/schema.py

@@ -1,5 +1,6 @@
 import binascii
 import datetime
+import django
 
 from django.db.backends.base.schema import (
     BaseDatabaseSchemaEditor,
@@ -677,7 +678,10 @@ def add_field(self, model, field):
         if self.connection.features.connection_persists_old_columns:
             self.connection.close()
 
-    def _create_unique_sql(self, model, columns, name=None, condition=None):
+    def _create_unique_sql(self, model, columns, name=None, condition=None, deferrable=None):
+        if (deferrable and not getattr(self.connection.features, 'supports_deferrable_unique_constraints', False)):
+            return None
+
         def create_unique_name(*args, **kwargs):
             return self.quote_name(self._create_index_name(*args, **kwargs))
 
@@ -687,22 +691,26 @@ def create_unique_name(*args, **kwargs):
         else:
             name = self.quote_name(name)
         columns = Columns(table, columns, self.quote_name)
+        statement_args = {
+            "deferrable": self._deferrable_constraint_sql(deferrable)
+        } if django.VERSION >= (3, 1) else {}
+
         if condition:
             return Statement(
                 self.sql_create_unique_index,
                 table=table,
                 name=name,
                 columns=columns,
                 condition=' WHERE ' + condition,
-                deferrable=''
+                **statement_args
             ) if self.connection.features.supports_partial_indexes else None
         else:
             return Statement(
                 self.sql_create_unique,
                 table=table,
                 name=name,
                 columns=columns,
-                deferrable=''
+                **statement_args
             )
 
     def _create_index_sql(self, model, fields, *, name=None, suffix='', using='',