The test command has not called sys.exit since commit
2c4fd43277fc477d85b50e15c37b176136676270.

Change coding cookie to use utf-8 (lowercase)

Provide a warning to users. Suggest using tox as an alternative generic
entry point.

Refs #1684

While perfectly valid, the encoding 'UTF-8' (uppercase) is not
recognized by the Emacs MULE system. As such, it displays the following
warning when opening a file with it used as an encoding cookie:

    Warning (mule): Invalid coding system ‘UTF-8’ is specified
    for the current buffer/file by the :coding tag.
    It is highly recommended to fix it before writing to a file.

Some discussion of this can be found at:

https://stackoverflow.com/questions/14031724/how-to-make-emacs-accept-utf-8-uppercase-encoding

While the post does offer a workaround for Emacs users, rather than ask
all to implement it, use the more typical utf-8 (lowercase).

Also reuse pre-defined py2_only and py3_only decorators where
appropriate.

- cleanup test: we're not installing, so no need to override the user site,
  or skip the test when run with a virtual environment
- use pytest support for capturing output (`context.quiet` does not work
  with Python 2), and check the output