Bump Python version

We are evaluating to generate Tor Browser offline documentation in our build pipeline (#34451 (closed)).

For this, we're using some of the repositories of the web team. They have a dependency on click==8.1.0, which is not compatible with Python 3.6.8, which is the Python version we're currently using.

In an IRC conversation I had this morning on #tor-www, emmapeel suggested me to stick with Python 3.8.x, because later versions have problems with i18n.

Notice that Firefox recommends a minimum version. As I wrote on #40478 (closed), 3.6.x is EOL, so we should be able to update to 3.8.x without any problems.