Add support for Python 3.8b4

Still needs the specific networking test classes added, but all the basics pass for me. Lets see about CI.

Dynamic class in gevent.resolver.thread.Resolver.__repr__

Fix references to six.PY3 in case python4 becomes a thing

monkey: allow to execute a module rather than only a script

API docs: remove duplicated entries for gevent.{queue,server}

Fixes: b7c9a1df ("Reorganize docs.")

The class name used in `gevent.resolver.thread.Resolver.__repr__` was
hard-coded. This meant that subclasses needed to reimplement their own
`__repr__`; otherwise, they'd appear as this parent class in debugging
output like that from print_run_info().

This also has the nice side effect of fixing a typo (`resolver_thread`
to `resolver.thread`).

Just based on https://github.com/ionelmc/python-tblib/commit/a96352760d20bedb881dd592fd342a732a36ec4b

Use CodeType.replace() if available

Port _tblib.Traceback to Python 3.8: use code.replace() method if
available.

The problems with memory fences that were referenced in 7f4e0b01 should be long gone: XCode has moved on, as has libev, and the conditionals in libev also check for __clang__ too, so it wasn't doing anything anymore.

Also update pyproject.toml to reflect what we're actually using version-wise, and add a note about why we use the build backend we do.

Make dnspython optional for testing.

[skip ci]