Move 3.9-dev to the 3.9 tag

After the feature freeze, master now points to 3.10.

plugins/python-build/share/python-build/3.9-dev

@@ -3,4 +3,4 @@ prefer_openssl11
 export PYTHON_BUILD_CONFIGURE_WITH_OPENSSL=1
 install_package "openssl-1.1.0j" "https://www.openssl.org/source/old/1.1.0/openssl-1.1.0j.tar.gz#31bec6c203ce1a8e93d5994f4ed304c63ccf07676118b6634edded12ad1b3246" mac_openssl --if has_broken_mac_openssl
 install_package "readline-8.0" "https://ftpmirror.gnu.org/readline/readline-8.0.tar.gz#e339f51971478d369f8a053a330a190781acb9864cf4c541060f12078948e461" mac_readline --if has_broken_mac_readline
-install_git "Python-3.9-dev" "https://github.com/python/cpython" master standard verify_py39 copy_python_gdb ensurepip
+install_git "Python-3.9-dev" "https://github.com/python/cpython" 3.9 standard verify_py39 copy_python_gdb ensurepip