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"`.

`setuptools` may potentially be needed for installation to work
fully as desired prior to Python 3.12, so in those versions it is
installed automatically in any virtual environment that is created
with `pip` available. This is a behavior of the `venv` module that
is not specific to CI.

However, on CI we upgrade packages that are preinstalled in the
virtual environment, or that we may otherwise wish to be present.
This took the form of unconditionally installing/upgrading the
`pip` and `wheel` packages, but conditionally upgrading the
`setuptools` package only if we find that it is already installed.

This commit changes the behavior to statically specify the same
list of package specifications to `pip` in all environments and in
all versions of Python, but to use the static notation recognized
by `pip` to indicate that `setuptools` is to be instaled/upgraded
only if the Python version is strictly less than Python 3.12.

This seems to be more readable. It also avoids using unquoted shell
parameter expansion in a way that is potentially confusing (for
example, if we were running our CI script steps through ShellCheck,
then it would automatically balk at that construction). It is also
more consistent with how `test_installation` sets up its
environment (especially since 31e1c03, but actually even before
that, because it was still conditioning `setuptools` on the Python
version rather than whether it was already installed). Finally,
this behavior is what the preexisting comment already described.

This also adjusts the shell quoting style slightly in other related
commands (in the same workflows) that pass package specifications
to `pip`, for consistency.

(For now, `".[test]"` rather than `.[test]` remains written in the
readme because it works in `cmd.exe` as well as other shells, but
it may be changed there in the future too.)