Fix #1888 (metadata accidentally not picklable), and removes a case where reimporting a vendored module results in a second copy of the same module.

Document support for Python 3.8 through a trove classifier

Remove outdated comment and suppressed exception from test_test.py

The test command has not called sys.exit since commit
2c4fd432.

Change coding cookie to use utf-8 (lowercase)

Deprecate the test command

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).

tests: drop (easy_install based) manual tests

minor cleanup

Small documentation tweaks

Crash when invalid python_requires indicated in setup.cfg

Honor declared ordering for Provides-Extra entries in package metadata.

launcher: Fix build with mingw-w64

Fix typo in CHANGES.rst

Fix some usage of deprecated `imp` module