Remove uses of warnings.catch_warnings (#2690)

[dairiki]

This removes the use of warnings.catch_warnings, thus preventing the rest of the global warnings registry (and the ensuing repetition of warnings filtered with the "default" action) as discussed at #2690.

• fixes Use of warnings.catch_warnings resets the global warnings registry #2690

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.
• Question (see below): Does werkzeug.wsgi._make_chunk_iter really have to issue a deprecation warning?

Question

Since _make_chunk_iter is not document API, can the deprecation warning it emits just be deleted? Doing so would remove several cases where catch_warnings was suppressing warnings from internal calls.