fix: clear all mypy errors

Annotate untyped containers and Optional attributes, type the stdlib-shaped
metric dicts in the client plugins, and add types-PyYAML/types-psutil to the
dev extras plus a mypy override for nio/mattermostdriver (no stubs).

Behaviour fixes found on the way: enable_kernel_timestamps() now returns False
when SO_TIMESTAMP is unknown instead of raising TypeError, send_shutdown is
defined at module level so cleanup() cannot hit a NameError, and a failed
subprocess returncode falls back to UNKNOWN.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

pyproject.toml

@@ -62,6 +62,8 @@ dev = [
   "pytest-cov>=4.0",
   "flake8>=5.0",
   "mypy>=1.10",
+  "types-PyYAML",
+  "types-psutil",
   "black>=23.0",
   "isort>=5.0",
   "tox>=4.0",
@@ -92,5 +94,10 @@ max-line-length = 111
 [tool.pylint.format]
 max-line-length = 111
 
+[[tool.mypy.overrides]]
+# optional notification libraries that ship neither stubs nor py.typed
+module = ["nio", "nio.*", "mattermostdriver", "mattermostdriver.*"]
+ignore_missing_imports = true
+
 [tool.pytest.ini_options]
 testpaths = ["tests"]