[Python] Change warnings to _warnings in _plasma_store_entry_point

[asfimport]

There is a leftover in python/pyarrow/__init__.py from #14343 due to warnings being imported as _warnings.

Connected GitHub issue: #14693

Reporter: Alenka Frim / @AlenkaF
Assignee: Alenka Frim / @AlenkaF

PRs and other links:

• GitHub Pull Request #14695

_{Note: This issue was originally created as ARROW-18379. Please see the migration documentation for further details.}

[asfimport]

Joris Van den Bossche / @jorisvandenbossche:
Issue resolved by pull request 14695
#14695