- don't compile Cython as recommended at https://cython.readthedocs.io/en/latest/src/quickstart/install.html
- don't install test extras we will not need

The coverage tracer breaks the test, which has a hard-coded list of functions it expects to find.

Because we seem to have extra greenlets.

We used to have this problem, but once again, I don't reproduce it on my mac.

20230707: Round-robin DNS on GitHub actions is constantly producing different addresses.

Update greentest to the versions we have on Github Actions. The versions on Appveyor may not yet match.

Prevents an issue with differing CPPFLAGS breaking configure when we try to compile again.

Fixes #1920

Update to greenlet 2.0

Request this for the two IDNA cases, which somehow give different errors using the same resolver on GHA.