When using DjangoDebugMiddleware, debug tracking presumes cursor.execute argument is a string, raising an error

* **What is the current behavior?**

When using the `DjangoDebugMiddleware`, Graphene Django wraps `cursor.execute` calls in a class that logs the query. Among the properties it logs is `is_select`, which attempts to guess if a query is a select query by looking for "select" in the argument to `cursor.execute`:
```
"is_select": sql.lower().strip().startswith("select"),
```

When `cursor.execute` is called with a non-string argument, such as a [psycopg2 query template]( https://www.psycopg.org/docs/sql.html#module-psycopg2.sql), this raises an error:
```python
from django.db import connection
from psycopg2 import sql

with connection.cursor() as cursor:
query = sql.SQL("SELECT * FROM {table}").format(table=sql.Identifier("my_table"))
cursor.execute(query)
```
This raises:
```
graphql.error.located_error.GraphQLLocatedError: 'Composed' object has no attribute 'lower'
```

* **What is the expected behavior?**
Grahene Django should not presume that the argument to `connection.cursor()` will always be a string, and do appropriate string coercion as needed before calling string methods on it.

* **What is the motivation / use case for changing the behavior?**
Robustness. It's ideal not to throw exceptions from normal uses of core Django methods which graphene-django monkeypatches.

* **Please tell us about your environment:**
- Version: graphene-django 2.5.0
- Platform: Ubuntu Linux 18.04, Django 2.2.12, graphene 2.1.8

Esamina DjangoDebugMiddleware, dove le chiamate a cursor.execute vengono avvolte e il valore di is_select viene calcolato dall'argomento SQL. Riproduci l'esempio psycopg2.sql.Composed dell'issue, quindi verifica che gli oggetti query non stringa non facciano più sollevare un errore al codice di tracking, mentre le query stringa mantengono il comportamento esistente.

Scritto dal modello di indicizzazione a partire dal testo della issue.