gh: upgrade to python 3.9 on Cygwin

Seem a new python37-sphinx release on Cygwin has broken dependencies, so
upgrate Cygwin test cases to python 3.9 which is the recommanded version.

.github/workflows/windows_tests.yaml

@@ -27,8 +27,8 @@ jobs:
       - name: Install system packages
         run: |
           # precise all sphinxcontrib packages required as dependency spec
-          # of python37-sphinx package is regularly broken
-          choco install make sed git autoconf automake libtool tcl python37 python37-sphinx zip python37-sphinxcontrib-applehelp python37-sphinxcontrib-devhelp python37-sphinxcontrib-htmlhelp python37-sphinxcontrib-jsmath python37-sphinxcontrib-qthelp python37-sphinxcontrib-serializinghtml python37-sphinxcontrib-websupport python37-alabaster python37-sqlalchemy --source cygwin
+          # of python39-sphinx package is regularly broken
+          choco install make sed git autoconf automake libtool tcl python39 python39-sphinx zip python39-sphinxcontrib-applehelp python39-sphinxcontrib-devhelp python39-sphinxcontrib-htmlhelp python39-sphinxcontrib-jsmath python39-sphinxcontrib-qthelp python39-sphinxcontrib-serializinghtml python39-sphinxcontrib-websupport python39-alabaster python39-sqlalchemy --source cygwin
       - name: Install Tcl under Windows
         shell: cmd
         run: |
@@ -71,8 +71,8 @@ jobs:
       - name: Install system packages
         run: |
           # precise all sphinxcontrib packages required as dependency spec
-          # of python37-sphinx package is regularly broken
-          choco install make sed git autoconf automake libtool tcl dejagnu python37 python37-sphinx ruby zsh tcsh mksh fish cmake R tcl-devel gcc-core wget python37-sphinxcontrib-applehelp python37-sphinxcontrib-devhelp python37-sphinxcontrib-htmlhelp python37-sphinxcontrib-jsmath python37-sphinxcontrib-qthelp python37-sphinxcontrib-serializinghtml python37-sphinxcontrib-websupport python37-alabaster python37-sqlalchemy --source cygwin
+          # of python39-sphinx package is regularly broken
+          choco install make sed git autoconf automake libtool tcl dejagnu python39 python39-sphinx ruby zsh tcsh mksh fish cmake R tcl-devel gcc-core wget python39-sphinxcontrib-applehelp python39-sphinxcontrib-devhelp python39-sphinxcontrib-htmlhelp python39-sphinxcontrib-jsmath python39-sphinxcontrib-qthelp python39-sphinxcontrib-serializinghtml python39-sphinxcontrib-websupport python39-alabaster python39-sqlalchemy --source cygwin
       - name: Build Modules
         run: |
           & $env:CYGWIN_BASH -lc 'cd $(cygpath -u "$GITHUB_WORKSPACE") && ./configure && make'