- Use importlib instead of imp

imp is deprecated and will go away in Python 3.12 (which is very
soon). The preferred replacement is importlib.

However, importlib is not available in Python 2.7, so we want
to keep both in order to maintain 2.7 some semblance of 2.7
support.

This implementation is based on the importlib docs [1] and passes
the quite robust tests that are already present for the urlparser
module. Unfortunately there's no good way to be 100% certain that
this works for all the many ways that Paste can do an import. So
this patch is mostly hoping that test coverage is sufficient.

[1] https://docs.python.org/3/library/importlib.html#importing-a-source-file-directly

Fixes #75

Borrowing the configuration from wsg-intercept and then editing
to fit so that we've got some automated tests.

To get this to work it was necessary to:

* adjust python versions
* update action version
* pin the ubuntu version
* fix warning in test_grantip
* form.cgi needs to not use six because it is not running in
   the virtualenv
* quiet deprecations in in form.cgi because cgi module is deprecated
* Skip the proxy test as it uses httpbin.org which is unreliable

This was done by adding a dictionary, called project_urls,
to the parameters of the setup function in setup.py

A Python 3 application might only define `__next__`, not `next`.  Use
`six.next` instead.

This is very similar to https://github.com/cdent/paste/pull/53, and was
apparently missed there.

* update tox to include py 39
* replace deprecated threading funcs