Add extra tests for weak references, and attempt a slightly-more-graceful failure mode than AttributeError when cyclic garbage collection is clearing gevent objects.

See issue #1961

This matches the standard library more closely.

Fixes #1932 and fixes #1931.

test__fileobject: Make readinto test use readable names; make readinto test not leak file descriptors; add change note for #1948.

Fixes #1948

.readinto was forgotten to be wrapped by FileObjectBase, and without the
wrapping FileObjectThread.readinto was calling into e.g.  RawIO.readinto
directly - instead of via threadpool - leading to deadlocks if e.g. that
readinto would need to read data from a pipe produced by another greenlet.

The fix is simple: add `readinto` into the list of wrapped methods in
FileObjectBase. Without the fix added `test_readinto` hangs for
TestFileObjectThread.

NOTE FileObjectPosix.readinto was working ok even without wrapping
because it implements cooperation by another way - via explicitly
`make_nonblocking` provided file descriptors.

/cc @jamadden

test__socket: Now that we are guaranteed to be using enums for .type and .family, strengthen the check from == to is.

Fixes #1950.

- 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.