[3.6] bpo-31627: Make test_mailbox be lenient to empty hostname. (GH-3821)

[miss-islington]

(cherry picked from commit f4ea642)

https://bugs.python.org/issue31627

[the-knights-who-say-ni]

Hello, and thanks for your contribution!

I'm a bot set up to make sure that the project can legally accept your contribution by verifying you have signed the PSF contributor agreement (CLA).

Unfortunately we couldn't find an account corresponding to your GitHub username on bugs.python.org (b.p.o) to verify you have signed the CLA (this might be simply due to a missing "GitHub Name" entry in your b.p.o account settings). This is necessary for legal reasons before we can look at your contribution. Please follow the steps outlined in the CPython devguide to rectify this issue.

Thanks again to your contribution and we look forward to looking at it!

[miss-islington]

@serhiy-storchaka: Backport status check is done, and it's a success ✅ .

[miss-islington]

Thanks, @serhiy-storchaka!