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

@@ -43,8 +43,10 @@ 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",
   "bidict>=0.23.1",