Migrate SDK HTTP stack from requests to httpx

[goncalossilva]

Summary

This PR migrates the SDK HTTP stack from requests to httpx across async AND sync clients.

What changed

• Replaced runtime HTTP client usage with httpx:
  • TodoistAPI(..., client: httpx.Client | None = None)
  • TodoistAPIAsync(..., client: httpx.AsyncClient | None = None)
• Removed executor-based async request wrappers from API I/O paths (true async HTTP calls now: closes Running Synchronous Code as Async Using Lambdas (Code Smell) #162).
• Migrated auth helpers to httpx (get_auth_token, revoke_auth_token, async variants).
• Updated low-level HTTP helpers (_core/http_requests.py) to httpx.
• Migrated tests from responses/requests-style mocks to respx.
• Added async-client lifecycle coverage and explicit async close guidance.
• Added shared constrained type aliases for API annotations.
• Updated docs/changelog for the migration.

Breaking changes

• session constructor arg is replaced by client (no compatibility alias):
  • TodoistAPI(session=...) -> TodoistAPI(client=...)
  • TodoistAPIAsync(session=...) -> TodoistAPIAsync(client=...)
• Authentication helpers now accept optional httpx.Client / httpx.AsyncClient instances instead of session: requests.Session.
• Exceptions to catch are now httpx.HTTPStatusError.

Next steps

Major version bump and release 4.0.0.

[Copilot]

Pull request overview

This PR successfully migrates the Todoist Python SDK's HTTP stack from requests to httpx, enabling true async HTTP I/O and removing the problematic executor-based async wrappers that blocked the event loop. The migration is comprehensive, covering both sync and async clients, authentication helpers, tests, and documentation.

Changes:

• Replaced requests with httpx for HTTP operations (sync: httpx.Client, async: httpx.AsyncClient)
• Removed executor-based async wrappers (run_async, generate_async) in favor of native async HTTP calls
• Migrated test mocking from responses to respx

Reviewed changes

Copilot reviewed 26 out of 27 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
uv.lock	Added httpx dependencies (httpx, httpcore, h11, anyio); removed requests and types-requests; replaced responses with respx
pyproject.toml	Updated dependencies: httpx>=0.28.1 instead of requests; respx instead of responses; removed types-requests
todoist_api_python/types.py	New shared type aliases module for ColorString, LanguageCode, ViewStyle (extracted from api.py)
todoist_api_python/_core/type_aliases.py	Re-exports type aliases from types.py for backwards compatibility
todoist_api_python/_core/utils.py	Removed run_async and generate_async functions; added kwargs_without_none helper
todoist_api_python/_core/http_requests.py	Complete rewrite using httpx.Client/AsyncClient; added true async variants (get_async, post_async, delete_async); uses httpx.Timeout instead of tuple
todoist_api_python/_core/http_headers.py	Removed Content-Type header creation (httpx handles this automatically with json= parameter)
todoist_api_python/api.py	Updated to use httpx.Client; replaced manual dict building with kwargs_without_none; removed noqa PLR0912 comments
todoist_api_python/api_async.py	Complete rewrite with true async HTTP; added proper client lifecycle management (aenter, aexit, close, del with ResourceWarning); uses AsyncIterator instead of AsyncGenerator; implements AsyncResultsPaginator
todoist_api_python/authentication.py	Migrated to httpx with context managers for client lifecycle; added async variants; uses _managed_client and _managed_async_client helpers
tests/*	Migrated all tests from responses to respx; updated mocking utilities; added async client lifecycle test; fixtures now use context managers
docs/*	Added async usage guidance; documented breaking changes; updated authentication examples
README.md	Added migration guide from 3.x to 4.x; documented async client lifecycle requirements
CHANGELOG.md	Documented all breaking changes for version 4.0.0

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[lefcha]

Overall looks good. I only had some comments about minor things (see inline comments).