Add support for Python 3.14

[hssyoo]

Add support for Python 3.14. Mostly chores:

• Add 3.14 to GH actions testing matrix
• Update README.md, setup.py, and tox.ini

It also replaces % with %% when adding AWS CLI command parameters to the arg parser. The latest versions of argparse added a check to raise exceptions when invalid formatted strings are passed to the help docs: python/cpython#124899. Some command parameters will fail this check because they have % characters in them, eg from aws s3api list-objects help:

NOTE:
     When using the URL encoding type, non-ASCII characters that are
     used in an object's key name will be percent-encoded according to
     UTF-8 code values. For example, the object test_file(3).png will
     appear as test_file%283%29.png .

argparse sees % and thinks it's a formatted string that needs to be expanded. And then it throws an exception when it fails to do so. We escape it with %% so it's interpreted as a literal %.

Verified that the bulit reference guide does not contain double %:

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.33%. Comparing base (68039fa) to head (8d5dca8).
⚠️ Report is 330 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #9790      +/-   ##
===========================================
- Coverage    93.40%   93.33%   -0.07%     
===========================================
  Files          211      209       -2     
  Lines        17012    16807     -205     
===========================================
- Hits         15890    15687     -203     
+ Misses        1122     1120       -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[chenrui333]

• there is some issue of building awscli 2.31.18 against py3.14, need Bump ruamel-yaml from 0.17.21 to 0.18.15 #9662

[chenrui333]

• Not sure about that the team thinks of this idea, Please update pinned versions #5943