Add asyncio support to the Python SDK

[kvz]

Why

Issue #9 asks for a Pythonic asyncio surface without changing sync callers. This PR adds a separate AsyncTransloadit API on top of the v2 runtime/security branch (#56) so async users can opt in without affecting the existing synchronous client.

What changed

• Added transloadit.async_client.AsyncTransloadit with async versions of the existing client helpers.
• Added async request/assembly/template helpers and a shared Response wrapper that works for both sync and async responses.
• Added async context manager cleanup plus explicit aclose()/close().
• Kept the sync Transloadit API untouched.

TUS/resumable tradeoff

• Non-resumable uploads use aiohttp directly.
• Resumable uploads still delegate to the existing tuspy uploader, but through asyncio.to_thread() so the event loop is not blocked. That keeps the implementation straightforward and honest about the sync TUS path.

Testing

• Added high-level async tests for GET/list/cancel/template flows, non-resumable assembly upload, wait=True polling with mocked asyncio.sleep, async context-manager cleanup, and resumable upload delegation.
• Verified locally with:
  • poetry check
  • poetry install
  • poetry run pytest tests
  • poetry run sphinx-build -E -b html docs/source docs/_build/html
  • poetry run tox -e py314
  • poetry build
  • bash -n scripts/test-in-docker.sh scripts/notify-registry.sh
  • git diff --check
  • ./scripts/test-in-docker.sh --python 3.12

This PR is intentionally based on #56 and targets base branch py310-v2.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.82904% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.52%. Comparing base (89a52c3) to head (7f8fc8c).

Files with missing lines	Patch %	Lines
transloadit/async_request.py	97.02%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           py310-v2      #57      +/-   ##
============================================
+ Coverage     86.60%   95.52%   +8.91%     
============================================
  Files             7       11       +4     
  Lines           209      625     +416     
  Branches         19       81      +62     
============================================
+ Hits            181      597     +416     
  Misses           28       28              

Flag	Coverage Δ
unittests	95.52% <98.82%> (+8.91%)	⬆️

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