Use static notation for setuptools in installation test

The `VirtualEnvironment` test helper is used in multiple places,
but it is only told to install/upgrade `pip` when used from
`test_installation`. It implements the functionality it would
ideally obtain by `upgrade_deps`, since the `upgrade_deps`
parameter is only avaiilable in `venv` when using Python 3.9 and
later.

When `pip` is installed, `upgrade_deps` would install `setuptools`
when using Python 3.11 or lower, but not when using Python 3.12 or
higher. `VirtualEnvironment` does the same. (The reason for this is
not just to avoid needlessly departing from what `upgrade_deps`
would do. Rather, it should not generally be necessary to have
`setuptools` installed for package management since Python 3.12,
and if it were necessary then this would a bug we would want to
detect while running tests.)

Previously this conditional specification of `setuptools` was done
by building different lists of package arguments to pass to `pip`,
by checking `sys.version_info` to decide whether to append the
string `setuptools`.

This commit changes how it is done, to use a static list of package
arguments instead. (The Python intepreter path continues to be
obtained dynamically, but all its positional arguments, including
those specifying packages, are now string literals.) The
conditional `setuptools` requirement is now expressed statically
using notation recognized by `pip`, as the string
`setuptools; python_version<"3.12"`.

test/lib/helper.py

@@ -415,9 +415,15 @@ def __init__(self, env_dir, *, with_pip):
 
         if with_pip:
             # The upgrade_deps parameter to venv.create is 3.9+ only, so do it this way.
-            command = [self.python, "-m", "pip", "install", "--upgrade", "pip"]
-            if sys.version_info < (3, 12):
-                command.append("setuptools")
+            command = [
+                self.python,
+                "-m",
+                "pip",
+                "install",
+                "--upgrade",
+                "pip",
+                'setuptools; python_version<"3.12"',
+            ]
             subprocess.check_output(command)
 
     @property