gh-117983: Defer import of threading for lazy module loading (#120233)

As noted in gh-117983, the import importlib.util can be triggered at
interpreter startup under some circumstances, so adding threading makes
it a potentially obligatory load.
Lazy loading is not used in the stdlib, so this removes an unnecessary
load for the majority of users and slightly increases the cost of the
first lazily loaded module.

An obligatory threading load breaks gevent, which monkeypatches the
stdlib. Although unsupported, there doesn't seem to be an offsetting
benefit to breaking their use case.

For reference, here are benchmarks for the current main branch:

```
❯ hyperfine -w 8 './python -c "import importlib.util"'
Benchmark 1: ./python -c "import importlib.util"
  Time (mean ± σ):       9.7 ms ±   0.7 ms    [User: 7.7 ms, System: 1.8 ms]
  Range (min … max):     8.4 ms …  13.1 ms    313 runs
```

And with this patch:

```
❯ hyperfine -w 8 './python -c "import importlib.util"'
Benchmark 1: ./python -c "import importlib.util"
  Time (mean ± σ):       8.4 ms ±   0.7 ms    [User: 6.8 ms, System: 1.4 ms]
  Range (min … max):     7.2 ms …  11.7 ms    352 runs
```

Compare to:

```
❯ hyperfine -w 8 './python -c pass'
Benchmark 1: ./python -c pass
  Time (mean ± σ):       7.6 ms ±   0.6 ms    [User: 5.9 ms, System: 1.6 ms]
  Range (min … max):     6.7 ms …  11.3 ms    390 runs
```

This roughly halves the import time of importlib.util.

1 files changed, 3 insertions, 1 deletions

diff --git a/Lib/importlib/util.py b/Lib/importlib/util.py
index 7243d05..8403ef9 100644
--- a/Lib/importlib/util.py
+++ b/Lib/importlib/util.py
@@ -13,7 +13,6 @@ from ._bootstrap_external import spec_from_file_location
 
 import _imp
 import sys
-import threading
 import types
 
 
@@ -257,6 +256,9 @@ class LazyLoader(Loader):
 
     def exec_module(self, module):
         """Make the module load lazily."""
+        # Threading is only needed for lazy loading, and importlib.util can
+        # be pulled in at interpreter startup, so defer until needed.
+        import threading
         module.__spec__.loader = self.loader
         module.__loader__ = self.loader
         # Don't need to worry about deep-copying as trying to set an attribute