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 90.38%. Comparing base (9ca599f) to head (55070e6).
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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #713   +/-   ##
=======================================
  Coverage   90.38%   90.38%           
=======================================
  Files          40       40           
  Lines        3546     3547    +1     
=======================================
+ Hits         3205     3206    +1     
  Misses        341      341           

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