assertion rewrite: use sys.stdlib_module_names to skip stdlib modules early, replacing the _in_find_spec flag

[Copilot]

With PYTHON_LAZY_IMPORTS=all (Python 3.15+), accessing a lazily-imported attribute inside find_spec triggers a recursive find_spec call. The previous fix used a _in_find_spec boolean guard that blocked all re-entrant calls globally.

Changes

• _early_rewrite_bailout: Add an early check against sys.stdlib_module_names (a built-in frozenset) to bail out immediately for stdlib modules — before calling fnmatch_ex, which is what triggers the lazy import resolution of fnmatch.fnmatch and causes the recursion. Stdlib modules are never assertion-rewritten anyway, so this is also a small performance win.
• find_spec: Remove the _in_find_spec boolean flag and the associated try/finally wrapper — they are no longer needed since the recursion is broken at the stdlib check in _early_rewrite_bailout.
• Tests: Replace the reentrancy-guard spy test with a direct assertion that find_spec returns None for stdlib modules (fnmatch, os, re).

The recursion chain that caused the crash:

find_spec("test_foo")
  → _early_rewrite_bailout("test_foo")
    → fnmatch_ex(pat, path)           # accesses fnmatch.fnmatch lazily
      → find_spec("fnmatch")          # triggered by lazy import resolution
        → _early_rewrite_bailout("fnmatch")
          → fnmatch_ex(…)             # infinite recursion

With the stdlib check, _early_rewrite_bailout("fnmatch") returns True immediately — breaking the cycle without any mutable per-call state.

[ofek]

@RonnyPfannschmidt Thanks for the suggestion! Hopefully this works 🙏

[sobolevn]

this is not technically required. on < 3.15, it does just does nothing. it can still be checked.

[RonnyPfannschmidt]

@soxofaan @ofek I'm on the playground with my daughter id appreciate if you guys gave this one a spin to see if it works in practice