[3.7] bpo-36520: Email header folded incorrectly (GH-13608) (GH-13910)

[3.7] bpo-36520: Email header folded incorrectly (GH-13608) (GH-13910)

* [bpo-36520](https://bugs.python.org/issue36520): reset the encoded word offset when starting a new
line during an email header folding operation

* 📜🤖 Added by blurb_it.

* [bpo-36520](https://bugs.python.org/issue36520): add an additional test case, and provide descriptive
comments for the test_folding_with_utf8_encoding_* tests

* [bpo-36520](https://bugs.python.org/issue36520): fix whitespace issue

* [bpo-36520](https://bugs.python.org/issue36520): changes per reviewer request -- remove extraneous
backslashes; add whitespace between terminating quotes and
line-continuation backslashes; use "bpo-" instead of
"issue GH-" in comments
(cherry picked from commit f6713e84afc5addcfa8477dbdf2c027787f711c0)

Co-authored-by: websurfer5 <49998481+websurfer5@users.noreply.github.com>
https://bugs.python.org/issue36520