workflows: use python 3.8
workflows: use python 3.8

With default python (3.6) we had this issue:
https://github.com/python/cpython/pull/13391

Signed-off-by: Stefano Garzarella <6f3dd2004310421c0dda1107f4857e62a774d5df@redhat.com>

stefanha/git-publish,stefanha/git-publish