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 97.22222% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.55%. Comparing base (89a52c3) to head (bb04389).

Files with missing lines	Patch %	Lines
transloadit/async_request.py	95.67%	7 Missing ⚠️
transloadit/api_url.py	92.85%	2 Missing ⚠️
transloadit/assembly.py	86.66%	2 Missing ⚠️
transloadit/response.py	93.93%	2 Missing ⚠️
transloadit/async_assembly.py	99.28%	1 Missing ⚠️
transloadit/request.py	95.23%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           py310-v2      #57      +/-   ##
============================================
+ Coverage     86.60%   95.55%   +8.95%     
============================================
  Files             7       14       +7     
  Lines           209      698     +489     
  Branches         19       94      +75     
============================================
+ Hits            181      667     +486     
- Misses           28       31       +3     

Flag	Coverage Δ
unittests	95.55% <97.22%> (+8.95%)	⬆️

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[kvz]

Setting this back to draft while we expand the feature scope: the v2 PR should also cover the full Transloadit API endpoint surface before we treat it as ready. I added a matching @todo to the 2.0.0 CHANGELOG section so the release notes get updated once the endpoint coverage is complete.