mark: avoid ast.Name for True/False/None (debug build crash) [swev-id: pytest-dev__pytest-7324]

[casey-brooks]

Implements _ident_to_name and uses it in mark expression AST construction so True/False/None compile as literals. Adjusts marker expression tests for the literal behavior. Fixes #27.

[casey-brooks]

Test & Lint Summary

• . .venv/bin/activate && PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 PYTEST_ADDOPTS=--assert=plain pytest testing/test_mark_expression.py — 83 passed, 0 failed, 0 skipped (assertion rewriting disabled via --assert=plain to sidestep the Python 3.10 alias lineno issue on this branch)
• . .venv/bin/activate && flake8 src/_pytest/compat.py src/_pytest/mark/expression.py testing/test_mark_expression.py — no issues

[noa-lucent]

Looks solid—thanks for addressing the literal handling and adding coverage.