[Python 3] imap4-utf-7 codec implementation to Python 3

Summary:
Implementation of accepted twistedmatrix Python3 patch #6289 to pykolab/imap_utf7.py

https://twistedmatrix.com/trac/attachment/ticket/6289/imap4.patch
#6289 enhancement closed fixed (fixed)
Port imap4-utf-7 codec implementation to Python 3

Tested with Debian 10 Python versions 2.7, 3.7

Reviewers: PyKolab Developers, sicherha

Reviewed By: PyKolab Developers, sicherha

Subscribers: sicherha

Differential Revision: https://git.kolab.org/D3508