chore(build): fix all mypy errors in setup.py

[vlad-scherbich]

Description

Fix a few lint errors in setup.py. Discovered when working on #17959.

Changes

• Add proper type annotations to LibraryDownload class attributes (name, version, url_root, available_releases, translate_suffix), resolving var-annotated and assignment errors in subclasses.
• Suppress cmake.CMAKE_BIN_DIR attr-defined errors (runtime-only attribute not declared in the cmake package stubs).
• Suppress method-assign errors for intentional debug-instrumentation monkey-patching of CustomBuildExt methods.
• Wrap src.suffix in bool() to fix return type mismatch in CMakeExtension.get_sources.

Testing

• commit passed pre-commit hooks
• CI

[cit-pr-commenter-54b7da]

Codeowners resolved as

setup.py                                                                @DataDog/python-guild

[pr-commenter]

Benchmarks

Benchmark execution time: 2026-05-07 19:54:40

Comparing candidate commit e6b1683 in PR branch vlad/fix-setup-py-mypy-errors with baseline commit 7834861 in branch main.

Found 0 performance improvements and 8 performance regressions! Performance is the same for 586 metrics, 4 unstable metrics.

scenario:iast_aspects-re_expand_aspect

• 🟥 execution_time [+253.666µs; +290.449µs] or [+7.283%; +8.339%]

scenario:iastaspects-casefold_noaspect

• 🟥 execution_time [+25.944µs; +30.085µs] or [+8.651%; +10.032%]

scenario:iastaspects-strip_aspect

• 🟥 execution_time [+60.971µs; +72.177µs] or [+22.573%; +26.722%]

scenario:iastaspects-upper_aspect

• 🟥 execution_time [+50.606µs; +56.350µs] or [+17.452%; +19.432%]

scenario:iastaspectsospath-ospathbasename_aspect

• 🟥 execution_time [+95.337µs; +101.764µs] or [+23.598%; +25.188%]

scenario:span-start

• 🟥 execution_time [+1.283ms; +1.448ms] or [+8.293%; +9.357%]

scenario:telemetryaddmetric-1-count-metric-1-times

• 🟥 execution_time [+201.490ns; +239.507ns] or [+9.718%; +11.552%]

scenario:tracer-small

• 🟥 execution_time [+26.323µs; +28.737µs] or [+7.551%; +8.243%]