Make various email.Policy use sites generic over the message type
#10678
| Job | Run time |
|---|---|
| 34s | |
| 36s | |
| 41s | |
| 35s | |
| 40s | |
| 35s | |
| 2m 14s | |
| 1m 31s | |
| 58s | |
| 1m 2s | |
| 1m 52s | |
| 2m 10s | |
| 58s | |
| 41s | |
| 44s | |
| 31s | |
| 26s | |
| 24s | |
| 17m 12s |