Prevent TypeError when bytes passed to cursor.execute in debug middleware

If DjangoDebugMiddleware is installed, calling `cursor.execute(b)` where b is a `bytes` object causes the recording (and thus the entire database call) to throw a TypeError due to https://github.com/graphql-python/graphene-django/blob/775644b5369bdc5fbb45d3535ae391a069ebf9d4/graphene_django/debug/sql/tracking.py#L126 :

```
"is_select": sql.lower().strip().startswith("select"),
```

Calling execute with a bytes parameter, to my knowledge, is not currently done within the high-level abstractions in the Django ORM, but is very much supported by psycopg2, as evidenced by the use in psycopg2's own `execute_values` in https://github.com/psycopg/psycopg2/blob/2_9_3/lib/extras.py#L1270 :

```
cur.execute(b''.join(parts))
```

This fix ensures that the sql parameter is safely decoded before scanning whether it begins with SELECT; since this is the only usage, the change is trivial.

The only workaround if code calls execute_values is to disable the DjangoDebugMiddleware altogether, which is far from ideal.

Author: bpartridge

graphene_django/debug/sql/tracking.py

@@ -110,6 +110,7 @@ def _record(self, method, sql, params):
             alias = getattr(self.db, "alias", "default")
             conn = self.db.connection
             vendor = getattr(conn, "vendor", "unknown")
+            sql_str = sql.decode(errors="ignore") if isinstance(sql, bytes) else sql
 
             params = {
                 "vendor": vendor,
@@ -123,7 +124,7 @@ def _record(self, method, sql, params):
                 "start_time": start_time,
                 "stop_time": stop_time,
                 "is_slow": duration > 10,
-                "is_select": sql.lower().strip().startswith("select"),
+                "is_select": sql_str.lower().strip().startswith("select"),
             }
 
             if vendor == "postgresql":