Use Python 3.8 on Cygwin CI

This uses Python 3.8.16 (provided by the Cygwin package python38 at
version 3.8.16-1), to work around the problem that pip has begun to
block on some PyPI package downloads when Python 3.9.18 (provided
by the Cygwin package python39 at version 3.9.18-1) is used.

I also tried a bunch of other stuff, which is listed below and can
be examined in full detail, with all individual diffs and most CI
results, at #2.

* Try not installing/upgrading wheel for Cygwin CI

This is for a recent problem where "pip install -U" in the virtual
environment in a Cygwin test job seems to block indefinitely on
downloading the wheel package itself (not other packages' wheels).

* Try not upgrading/installing pip/setuptools either on Cygwin

* Try installing pytho39-wheel Cygwin package

Maybe this will overcome the next blockage, which is the codecov
PyPI package, downloading a .tar.gz file.

* Try upgrading wheel, but after upgrading pip

* Try always running pip on Cygwin as "python -m pip"

* Try using a venv on Cygwin

* Use "python -v -m pip" to see some of what's going on

* Undo venv; use "python -m pip -vvv" to see what's going on

* Undo all debugging changes except passing "-vvv"

* Try with "--no-cache-dir"

* Try with different tmp dir for pip runs

* Try with python39=3.9.16-1

* Try not upgrading setuptools

* Try not installing Cygwin python39-pip package

* Run pip freeze effectively

This doesn't fix the bigger issue, it just addresses something from
the last commit.

* Try not installing python39-virtualenv either

* Try giving IPv4 for files.pythonhosted.org in hosts file

* Try downloading wheel with wget

This is not a usable solution, but it is useful for troubleshooting.

* Try with python39-pip=23.0.1-1

And don't upgrade it or other PyPI packages.

* Pin pip with pip (Cygwin package doesn't pin)

This tries with an older pip, but if the problem is the build
rather than the version, then it would also help.

* Stop pinning; keep skipping -U for PyPA; instrument with -vvv

This won't fix it but is diagnostic, to reveal the URL for the
coverage package, so I can see what happens when that is installed
more manually.

* Try installing coverage[toml] separately

* Unset -vvv to see the bigger picture more easily

* Try killing pip after a timeout and rerunning it

* Use SIGKILL

* Increase timeout from 70 to 120 seconds per try

* Give each try a little more time than the last

Since it has to verify previous work.

* Tweak (re)try parameters

* Try Python 3.8

.github/workflows/cygwin-test.yml

@@ -30,7 +30,7 @@ jobs:
     - name: Install Cygwin
       uses: cygwin/cygwin-install-action@v4
       with:
-        packages: python39 python39-pip python39-virtualenv git
+        packages: python38 python38-pip python38-virtualenv git
         add-to-path: false  # No need to change $PATH outside the Cygwin environment.
 
     - name: Arrange for verbose output
@@ -55,28 +55,23 @@ jobs:
         # and cause subsequent tests to fail
         cat test/fixtures/.gitconfig >> ~/.gitconfig
 
-    - name: Ensure the "pip" command is available
-      run: |
-        # This is used unless, and before, an updated pip is installed.
-        ln -s pip3 /usr/bin/pip
-
     - name: Update PyPA packages
       run: |
-        # Get the latest pip, wheel, and prior to Python 3.12, setuptools.
-        python -m pip install -U pip $(pip freeze --all | grep -ow ^setuptools) wheel
+        # Get the latest pip, setuptools, and wheel.
+        python3.8 -m pip install -U pip setuptools wheel
 
     - name: Install project and test dependencies
       run: |
-        pip install ".[test]"
+        python3.8 -m pip install ".[test]"
 
     - name: Show version and platform information
       run: |
         uname -a
-        command -v git python
+        command -v git python3.8
         git version
-        python --version
-        python -c 'import os, sys; print(f"sys.platform={sys.platform!r}, os.name={os.name!r}")'
+        python3.8 --version
+        python3.8 -c 'import os, sys; print(f"sys.platform={sys.platform!r}, os.name={os.name!r}")'
 
     - name: Test with pytest
       run: |
-        pytest --color=yes -p no:sugar --instafail -vv
+        python3.8 -m pytest --color=yes -p no:sugar --instafail -vv