Use python39-pip-wheel Cygwin package on CI

Instead of downloading and running the boostrap script.

Various Cygwin packages have been tried here for gitpython-developers#2004, but I don't
recall having explicitly specified `python39-pip-wheel`. On the
Cygwin mailing list, Daniel Abrahamsson reported a problem that
seems to be the same as encountered here, and noted that installing
`python39-pip-wheel` fixed it. So maybe that will help here.

This commit also delists the `python39-virtualenv` pacakge, which
it looks like we have not been using (anytime recently) because it
actually provides the `virtualenv` module and command, while we are
using `venv` from the Python standard library, including on Cygwin.

Author: EliahKagan

Diff for: .github/workflows/cygwin-test.yml

@@ -30,7 +30,7 @@ jobs:
     - name: Install Cygwin
       uses: cygwin/cygwin-install-action@v5
       with:
-        packages: python39 python39-pip python39-virtualenv git wget
+        packages: python39 python39-pip-wheel
         add-to-path: false  # No need to change $PATH outside the Cygwin environment.
 
     - name: Arrange for verbose output
@@ -58,13 +58,9 @@ jobs:
 
     - name: Set up virtualenv
       run: |
-        python3.9 -m venv --without-pip .venv
+        python3.9 -m venv .venv
         echo 'BASH_ENV=.venv/bin/activate' >>"$GITHUB_ENV"
 
-    - name: Bootstrap pip in virtualenv
-      run: |
-        wget -qO- https://bootstrap.pypa.io/get-pip.py | python
-
     - name: Update PyPA packages
       run: |
         # Get the latest pip, wheel, and prior to Python 3.12, setuptools.