fix type of sendgrid sandbox mode

oser_backend/oser_backend/settings/production.py

@@ -23,8 +23,12 @@ SECURE_BROWSER_XSS_FILTER = True
 
 # Mails
 MAILS_ENABLED = True
-SENDGRID_SANDBOX_MODE_IN_DEBUG = os.environ.get(
-    'SENDGRID_SANDBOX_MODE_IN_DEBUG', True)
+
+# Sendgrid
+sandbox_mode = os.environ.get('SENDGRID_SANDBOX_MODE_IN_DEBUG', True)
+if sandbox_mode in ('0', 'False', 'false'):
+    sandbox_mode = False
+SENDGRID_SANDBOX_MODE_IN_DEBUG = bool(sandbox_mode)
 
 # Celery settings