[Feature] Support automatic showing of warnings after SQL execution (#555)

[scottnemes]

Description

Adds support for the automatic displaying of warnings after a SQL statement is executed. Addresses feature request from issue #555.

May be set with:

• Commands \W and \w
• In the config file with show_warnings
• With --show-warnings/--no-show-warnings on the command line

Note there is currently some errors from mypy about Cursor.warning_count, however I already submitted a PR to typeshed to get that updated so the errors will go away once the change makes it into mypy for types-PyMySQL.
python/typeshed#15150 (comment)

Checklist

• I've added this contribution to the changelog.md.
• I've added my name to the AUTHORS file (or it's already there).
• I ran uv run ruff check && uv run ruff format && uv run mypy --install-types . to lint and format the code.

[rolandwalker]

Outstanding! Looks like the type stubs will be updated tomorrow.

[your-diary]

For those who read the release note of v1.42.0 to find this functionality and tried to use SELECT INTO @var with --show-warnings but got no automatic warning display, there is a follow-up PR to complete the feature that has NOT been released yet.

The PR says

Currently the new automatic show_warnings logic does not work for queries that return zero results, like DDL.

If you want to test the functionality in v1.42.0, try this:

SELECT 1 + '0 foo';