chore: minor cleanups and perf tweaks from code review (#2910)

* chore: minor cleanups and perf tweaks from code review

- util/file.py, util/python_build_standalone.py: use hashlib.file_digest
  for streaming SHA-256 verification instead of loading whole archives
  into memory with read_bytes()
- logger.py: convert colors/symbols properties to functools.cached_property
  so the Colors/Symbols objects are constructed only once per Logger instance
- platforms/windows.py: remove redundant .strip() on where_pip (already
  stripped at assignment)
- util/python_build_standalone.py: remove unreachable python_base_dir.exists()
  guard (callers always pass a fresh temp subdirectory)
- util/file.py: add comment explaining the getattr shim for
  tar_.extraction_filter and when it can be removed

Assisted-by: ClaudeCode:claude-fable-5

* revert: restore assertion to check python_base_dir existence

cibuildwheel/platforms/windows.py

@@ -348,7 +348,7 @@ def setup_python(
         assert (venv_path / "Scripts" / "pip.exe").exists()
         where_pip = call("where", "pip", env=env, capture_stdout=True).splitlines()[0].strip()
         print(where_pip)
-        if where_pip.strip() != str(venv_path / "Scripts" / "pip.exe"):
+        if where_pip != str(venv_path / "Scripts" / "pip.exe"):
             msg = "pip available on PATH doesn't match our installed instance. If you have modified PATH, ensure that you don't overwrite cibuildwheel's entry or insert pip above it."
             raise errors.FatalError(msg)
         call("pip", "--version", env=env)