build(python): bump floor from 3.10 to 3.12

Drops Python 3.10 and 3.11 support across packaging, lint, type checking,
and CI test matrix. Unblocks the upcoming Teams adapter migration to
microsoft-teams-apps (#93), which requires Python 3.12+. Our primary
consumer (chinchill-api) already runs on 3.12, so no consumer impact.

Changes:
- pyproject.toml: requires-python >=3.12, ruff target-version py312,
  pyrefly python-version 3.12, drop 3.10/3.11 classifiers
- .github/workflows/test.yml: matrix now ["3.12", "3.13"]
- README.md: Python 3.10+ -> 3.12+ in feature comparison

Ruff modernization (UP017 datetime.UTC, UP040 PEP 695 type, UP041
TimeoutError) is intentionally ignored for now — this PR is the floor
bump only, not the codebase modernization. Those cleanups can land
incrementally as code changes naturally touch the affected call sites.

Tracking: #98 (Python 3.12 sync wave), #93 (Teams SDK migration).

https://claude.ai/code/session_01FyMxQn2BEAzmwKS1GZczKj

Author: claude

.github/workflows/test.yml

@@ -14,7 +14,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.10", "3.11", "3.12", "3.13"]
+        python-version: ["3.12", "3.13"]
     steps:
       - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5  # v4.2.2
       - uses: astral-sh/setup-uv@e4db8464a088ece1b920f60402e813ea4de65b8f  # v4

README.md

@@ -71,7 +71,7 @@ async def handle_mention(thread, message):
 | Feature | chat-sdk | Raw platform SDKs | BotFramework SDK |
 |---------|----------|--------------------|------------------|
 | Multi-platform from one codebase | 8 platforms | 1 per SDK | Teams + limited |
-| Async-native (Python 3.10+) | Yes | Varies | No |
+| Async-native (Python 3.12+) | Yes | Varies | No |
 | Cross-platform cards | Card model | Platform-specific | Adaptive Cards only |
 | Thread locking / dedup | Built-in | DIY | DIY |
 | State abstraction (mem/redis/pg) | Built-in | DIY | DIY |

pyproject.toml

@@ -18,16 +18,14 @@ keywords = [
 ]
 readme = "README.md"
 license = {text = "MIT"}
-requires-python = ">=3.10"
+requires-python = ">=3.12"
 dependencies = []
 classifiers = [
     "Development Status :: 3 - Alpha",
     "Framework :: AsyncIO",
     "Intended Audience :: Developers",
     "License :: OSI Approved :: MIT License",
     "Programming Language :: Python :: 3",
-    "Programming Language :: Python :: 3.10",
-    "Programming Language :: Python :: 3.11",
     "Programming Language :: Python :: 3.12",
     "Programming Language :: Python :: 3.13",
     "Topic :: Communications",
@@ -99,11 +97,19 @@ include = [
 asyncio_mode = "auto"
 
 [tool.ruff]
-target-version = "py310"
+target-version = "py312"
 line-length = 120
 
 [tool.ruff.lint]
 select = ["E", "F", "I", "B", "SIM", "UP"]
+# Modernization of pre-3.12 patterns is deferred to follow-up PRs; the 3.12
+# floor bump itself doesn't rewrite call sites. Re-enable these as the
+# codebase is migrated.
+ignore = [
+    "UP017",  # datetime.timezone.utc → datetime.UTC (3.11+)
+    "UP040",  # TypeAlias → PEP 695 `type` keyword (3.12+)
+    "UP041",  # asyncio.TimeoutError → builtins TimeoutError (3.11+)
+]
 
 [dependency-groups]
 dev = [
@@ -132,7 +138,7 @@ project-excludes = ["**/__pycache__", "**/.pytest_cache"]
 disable-project-excludes-heuristics = true
 use-ignore-files = false
 
-python-version = "3.10"
+python-version = "3.12"
 python-platform = "linux"
 
 # Optional adapter deps that aren't in the default install. Treating them as