#20974: Update version table in email intro.

Note that the email version string in 3.3 and 3.4 is inaccurate, since it
still exists but wasn't updated while the code was.
Issue #16537: Check whether self.extensions is empty in setup.py.

Patch by Jonathan Hosmer.
#18854: make it explicit that is_multipart does not mean 'multipart/xxx'.

Original patch (and the idea of using _structure) by Abhilash Raj.
Merge heads.