Fix encoding warnings in tests.

[jaraco]

• In test_build_ext
• In support
• In test_build_py
• Specify encoding in util.byte_compile. Ref 150 warnings #232.
• Rely on tree builder in test_build_scripts. Ref 150 warnings #232.
• Rely on Path object to replace the suffix
• Fix EncodingWarnings in test_core. Ref 150 warnings #232.
• Ran pyupgrade for Python 3.8+ followed by ruff format.
• Rely on tree builder in test_dir_util. Ref 150 warnings #232.
• Rely on tree builder and path objects. Ref 150 warnings #232.
• Remove reliance on TempdirManager in test_file_util.
• Rely on tmp_path fixture directly.
• 👹 Feed the hobgoblins (delint).
• Rely on tree builder. Ref 150 warnings #232.
• Specify encoding in test_install. Ref 150 warnings #232.
• Re-use write_sample_scripts in test_install_scripts. Ref 150 warnings #232.
• Use Path objects in test_register. Ref 150 warnings #232.
• Specify encoding in test_sdist. Ref 150 warnings #232.
• Fix EncodingWarning in test_spawn. Ref 150 warnings #232.
• Fix EncodingWarnings in test_sdist. Ref 150 warnings #232.
• Rely on tree builder. Ref 150 warnings #232.
• Suppress diffcov error.