Split py-version-gated uv dependency-groups

Reshuffle `pyproject.toml` deps into per-python-version
`[tool.uv.dependency-groups]`:
- `subints` group: `msgspec>=0.21.0`, py>=3.14
- `eventfd` group: `cffi>=1.17.1`, py>=3.13,<3.14
- `sync_pause` group: `greenback`, py>=3.13,<3.14
  (was in `devx`; moved out bc no 3.14 yet)

Bump top-level `msgspec>=0.20.0` too.

(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

(cherry picked from commit 34d9d482e4)
(factored: kept only the pyproject dep-group parts of
 "Raise `subint` floor to py3.14 and split dep-groups"; dropped
 tractor/spawn/_spawn.py + tractor/spawn/_subint.py)

pyproject.toml

@@ -44,11 +44,12 @@ dependencies = [
   "tricycle>=0.4.1,<0.5",
   "wrapt>=1.16.0,<2",
   "colorlog>=6.8.2,<7",
+
   # built-in multi-actor `pdb` REPL
   "pdbp>=1.8.2,<2", # windows only (from `pdbp`)
+
   # typed IPC msging
   "msgspec>=0.20.0",
-  "cffi>=1.17.1",
   "bidict>=0.23.1",
   "multiaddr>=0.2.0",
   "platformdirs>=4.4.0",
@@ -64,10 +65,13 @@ dev = [
 ]
 devx = [
   # `tractor.devx` tooling
-  "greenback>=1.2.1,<2",  # TODO? 3.14 greenlet on nix?
   "stackscope>=0.2.2,<0.3",
   # ^ requires this?
   "typing-extensions>=4.14.1",
+  # {include-group = 'sync_pause'},  # XXX, no 3.14 yet!
+]
+sync_pause = [
+  "greenback>=1.2.1,<2",  # TODO? 3.14 greenlet on nix?
 ]
 testing = [
   # test suite
@@ -85,6 +89,14 @@ repl = [
 lint = [
   "ruff>=0.9.6"
 ]
+# XXX, used for linux-only hi perf eventfd+shm channels
+# now mostly moved over to `hotbaud`.
+eventfd = [
+  "cffi>=1.17.1",
+]
+subints = [
+  "msgspec>=0.21.0",
+]
 # TODO, add these with sane versions; were originally in
 # `requirements-docs.txt`..
 # docs = [
@@ -93,6 +105,17 @@ lint = [
 # ]
 # ------ dependency-groups ------
 
+[tool.uv.dependency-groups]
+# for subints, we require 3.14+ due to 2 issues,
+# - hanging behaviour for various multi-task teardown cases (see
+#   "Availability" section in the `tractor.spawn._subints` doc string).
+# - `msgspec` support which is oustanding per PEP 684 upstream tracker:
+#   https://github.com/jcrist/msgspec/issues/563
+#
+# https://docs.astral.sh/uv/concepts/projects/dependencies/#group-requires-python
+subints = {requires-python = ">=3.14"}
+eventfd = {requires-python = ">=3.13, <3.14"}
+sync_pause = {requires-python = ">=3.13, <3.14"}
 
 [tool.uv.sources]
 # XXX NOTE, only for @goodboy's hacking on `pprint(sort_dicts=False)`