cygwin: fix python versions again

Now cygwin seems to have completed a migration of the default python to
3.9, so that is where the devel package is at.

Back out the changes from commit 3304a38496
and update the pip/wheel packages as appropriate.

.github/workflows/cygwin.yml

@@ -66,18 +66,11 @@ jobs:
             python3-devel
             python3-libxml2
             python3-libxslt
-            python38-pip
-            python38-wheel
+            python39-pip
+            python39-wheel
             vala
             zlib-devel
 
-      - name: workaround wrong python version
-        run: |
-          export PATH=/usr/sbin:/usr/bin:/usr/local/bin:$(cygpath ${SYSTEMROOT})/system32
-          update-alternatives --verbose --set python /usr/bin/python3.8
-          update-alternatives --verbose --set python3 /usr/bin/python3.8
-        shell: C:\cygwin\bin\bash.exe --noprofile --norc -o igncr -eo pipefail '{0}'
-
       - name: Run pip
         run: |
           export PATH=/usr/bin:/usr/local/bin:$(cygpath ${SYSTEMROOT})/system32