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.

Its failing in a weird way that doesn't happen locally.

There is a remaining issue, which lies somewhere in greenlet, seen in
test__monkey_hub_in_thread and test__threading_monkey_in_thread:

  Fatal Python error: Python memory allocator called without holding the GIL

  Python runtime state: finalizing (tstate=0xd5f3e0)

  Thread 0x00007f511f66b700 (most recent call first):
  <no Python frame>

Obviously this mixes with threads. greenlet uses Py_AddPendingCall to
move some work to other threads, but apparently something is still
happening at the wrong time.

Fixes #1909

We'll need to release a final greenlet 2.0 and remove the rc qualifiers here before releasing gevent, because pre-release qualifiers are contagious.

Stop using DEF statements in Cython, as they are deprecated.

See https://github.com/cython/cython/issues/4310