Have test_installation test that operations produce no warnings

By setting the `PYTHONWARNINGS` environment variable to `error` in
each of the subprocess invocations.

This is strictly stronger than passing `-Werror` for the `python`
commands, because it automatically applies to subprocesses (unless
they are created with a sanitized environment or otherwise with one
in which `PYTHONWARNINGS` has been customized), and because it
works for `pip` automatically.

Importantly, this will cause warnings internal to Python
subprocesses created by `pip` to be treated as errors. It should
thus surface any warnings coming from the `setuptools` backend.

Author: EliahKagan

test/test_installation.py

@@ -64,6 +64,7 @@ def _set_up_venv(rw_dir):
             subprocess.run,
             stdout=subprocess.PIPE,
             cwd=venv.sources,
+            env={**os.environ, "PYTHONWARNINGS": "error"},
         )
 
         return venv, run