[issue36303] Trying to change values (fe. "To", "From") of email.mime.text.MIMEText after initial assigment silently doesn't change them.

[Łukasz]

New submission from Łukasz <fotobl...@fejm.pl>:

Script reproducing this behavior:

```python3
from email.mime.text import MIMEText
msg = MIMEText('<div/>', 'html')
msg["To"] = "this.email.shouldnt.be.prin...@nokia.com"
msg["To"] = "valid.em...@nokia.com"
print(msg["To"])
```

stdout:
```
this.email.shouldnt.be.prin...@nokia.com
```

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<https://bugs.python.org/issue36303>
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