upgrade Pex to 2.1.161

Changelogs:
 * https://github.com/pantsbuild/pex/releases/tag/v2.1.160
 * https://github.com/pantsbuild/pex/releases/tag/v2.1.161

```
Lockfile diff: 3rdparty/python/user_reqs.lock [python-default]

==                    Upgraded dependencies                     ==

  cryptography                   41.0.7       -->   42.0.1
  pex                            2.1.159      -->   2.1.161
  pluggy                         1.3.0        -->   1.4.0
  pydantic                       1.10.13      -->   1.10.14
  python-dotenv                  1.0.0        -->   1.0.1
```

Further suport relative to pantsbuild#15704

3rdparty/python/requirements.txt

@@ -10,7 +10,7 @@ fasteners==0.16.3
 freezegun==1.2.1
 ijson==3.1.4
 packaging==21.3
-pex==2.1.159
+pex==2.1.161
 psutil==5.9.0
 # This should be compatible with pytest.py, although it can be looser so that we don't
 # over-constrain pantsbuild.pants.testutil