Handle py3.14+ incompats as test skips

Since we're devving subints we require the 3.14+ stdlib API
and a couple compiled libs don't support it yet, namely:
- `cffi`, which we're only using for the `.ipc._linux` eventfd
  stuff (now factored into `hotbaud` anyway).
- `greenback`, which requires `greenlet` which doesn't seem to be
  wheeled yet
  * on nixos the sdist build was failing due to lack of `g++` which
    i don't care to figure out rn since we don't need `.devx` stuff
    immediately for this subints prototype.
  * [ ] we still need to adjust any dependent suites to skip.

Adjust `test_ringbuf` to skip on import failure.

Also project wide,
- pin us to py 3.13+ in prep for last-2-minor-version policy.
- drop `msgspec>=0.20.0`, the first release with py3.14 support.

(cherry picked from commit d2ea8aa2de)

tests/test_ringbuf.py

@@ -21,6 +21,9 @@ from tractor._testing.samples import (
 # XXX, in case you want to melt your cores, comment this skip line XD
 pytestmark = pytest.mark.skip
 
+# XXX `cffi` dun build on py3.14 yet..
+cffi = pytest.importorskip("cffi")
+
 
 @tractor.context
 async def child_read_shm(

tractor/ipc/_linux.py

@@ -17,10 +17,20 @@
 Linux specifics, for now we are only exposing EventFD
 
 '''
-import os
 import errno
+import os
+import sys
+
+try:
+    import cffi
+except ImportError as ie:
+    if sys.version_info < (3, 14):
+        ie.add_note(
+            f'The `cffi` pkg has no 3.14 support yet.\n'
+        )
+
+    raise ie
 
-import cffi
 import trio
 
 ffi = cffi.FFI()