[v2] Add support for Python 3.14

[hssyoo]

Addresses #9914

Changes:

• Port test fixes from Test on 3.14 beta boto/botocore#3450 and Add official Python 3.14 support with first release candidate boto/botocore#3522
• Set EKS YAML object width to inf to avoid adding unnecessary line breaks when dumping. This maintains roundtrip and prevents existing tests from failing
• Fix local URI construction when installing downloaded packages in build script
• Update finally clause to comply with PEP 765
• Add Python 3.14 to GitHub Actions test matrix
• Update all Python 3.13 settings to 3.14
• Bump pinned pip build dependency to 26.0.1
• Bump flit_core ceiling to 3.12.0
• Bump ruamel.yaml ceiling to 0.19.1
• Bump ruamel.yaml.clib ceiling to 0.2.15
• Bump pyinstaller to 6.19.0
• Pin virtualenv dev dependency to 20.35.4 to avoid conditional Python version ranges