fix: replace invalid tuple type annotations with typing.Tuple

[SukhmanH]

Closes #540

Proposed Changes

Briefly describe your proposed changes:

The batching callback signatures annotate conf as (str, str, str). A tuple of
types is not a valid type annotation, so any code copy-pasted from the README
fails type checking:

example.py:7: error: Syntax error in type annotation  [syntax]
example.py:7: note: Suggestion: Use Tuple[T1, ..., Tn] instead of (T1, ..., Tn)

This replaces the pattern with Tuple[str, str, str] in the 23 places it
appears:

• influxdb_client/client/write_api.py — the _BatchItem.to_key_tuple() return
  annotation, which is the value actually passed to the callbacks
  (bucket, org, precision)
• influxdb_client/client/util/multiprocessing_helper.py — the
  _success_callback / _error_callback / _retry_callback signatures
• influxdb_client/client/influxdb_client.py and multiprocessing_helper.py
  docstring examples
• examples/write_api_callbacks.py, examples/http_error_handling.py
• tests/test_WriteApiBatching.py
• README.md

typing.Tuple is used rather than the PEP 585 builtin tuple[...] because
setup.py still declares python_requires='>=3.7'. The copy-pasteable examples
also gained the matching from typing import Tuple import so they remain
runnable as-is, which was the original point of the issue.

This is annotation-only — no runtime behaviour changes. Verified:

• mypy on the README snippet: 3 errors before, clean after
• flake8 setup.py influxdb_client/ reports the same 2 pre-existing E721
  warnings in generated _sync/_async code as on master, and nothing new
• pytest tests/test_WriteApiBatching.py — 24 passed

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• pytest tests completes successfully
• Commit messages are conventional
• Sign CLA (if not already signed)

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.41%. Comparing base (97b0209) to head (38d8241).
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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #713   +/-   ##
=======================================
  Coverage   90.41%   90.41%           
=======================================
  Files          40       40           
  Lines        3546     3547    +1     
=======================================
+ Hits         3206     3207    +1     
  Misses        340      340           

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[karel-rehor]

@SukhmanH

Thank you for your contribution!

All tests are stable.

N.B. in the CHANGELOG.md we need the link to the PR that resolves the issue not the link to the issue. I see this was not detected in previous PR merges. I'll fix this up in a separate PR before the release.

Looks good to me. 🚴 🏁