Import smtp in the executor to avoid blocking the loop (#112089)

This one has no external requirements, but the email imports
from stdlib take ~0.6s on a green

homeassistant/components/smtp/manifest.json

@@ -3,5 +3,6 @@
   "name": "SMTP",
   "codeowners": [],
   "documentation": "https://www.home-assistant.io/integrations/smtp",
+  "import_executor": true,
   "iot_class": "cloud_push"
 }