Fix negative timezone bug introduced when migrating to python 3

[peterb-dev]

The timezone's negative sign must be checked against a byte-string, rather than the python 3 unicode string. Way back in 2011 this bug was fixed in what is likely the original sourcecode for Internaldate2epoch(), see: python/cpython#55148

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This PR

Add character x [x].

• I've read the DCO.
• I've read the Coding Guidelines
• The relevant informations about the changes stands in the commit message, not here in the message of the pull request.
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• Code is tested (provide details).

References

• Issue IMAP <> IMAP sync introduces a large fixed offset for the received date-time when mail headers contain a negative timezone (eg a USA-based remote mail host). #185

Additional information

[etbuira]

Looks good to me.