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]

Fix caching for the non-embedded cases; The embedded cases don't work because of pip's use of isolation into random temp directories, which may also be impacting ccache's effectiveness.

[appveyor skip]

[appveyor skip]

Clean up configuring for embedded CFFI modules.

Fixes #1411.

Make the CFFI backends work with non-embedded libev/libuv

Fixes #1395

Rework testrunner and tests to support resources and disabling network tests

Fixes #1349 and fixes #1402

Deprecate the old EMBED vars in favor of GEVENTSETUP_EMBED.

Refs #1402

Also documented the extras, and added a simpler one.