[docs] [issue20477] Add examples of using the new contentmanager api to the email docs

[Roundup Robot]

Roundup Robot added the comment:

New changeset a9d7d53d5fbd by R David Murray in branch 'default':
#20477: add examples of using the new contentmanager API.
http://hg.python.org/cpython/rev/a9d7d53d5fbd

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nosy: +python-dev

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