hammer: FreeBSD 13.0 installation fails

An attempt to use hammer to install FreeBSD fails, because py38-sphinx and py38-sphinx_rtd_theme are no longer available. Need to migrate to 3.9.

Steps to reproduce:

./hammer.py prepare-system -p virtualbox -s freebsd -r 13.0 -l

Result:

Updating FreeBSD repository catalogue...
FreeBSD repository is up to date.
All repositories are up to date.
pkg: No packages available to install matching 'py38-sphinx' have been found in the repositories
pkg: No packages available to install matching 'py38-sphinx_rtd_theme' have been found in the repositories
Traceback (most recent call last):
  File "/home/vagrant/hammer.py", line 3058, in <module>
    main()
  File "/home/vagrant/hammer.py", line 3030, in main
    prepare_system_cmd(args)
  File "/home/vagrant/hammer.py", line 2860, in prepare_system_cmd
    prepare_system_local(features, args.check_times)
  File "/home/vagrant/hammer.py", line 1803, in prepare_system_local
    install_pkgs(packages, env=env, timeout=6 * 60, check_times=check_times)
  File "/home/vagrant/hammer.py", line 515, in install_pkgs
    execute(cmd, timeout=timeout, env=env, check_times=check_times, attempts=3, sleep_time_after_attempt=10)
  File "/home/vagrant/hammer.py", line 401, in execute
    raise ExecutionError("The command return non-zero exitcode %s, cmd: '%s'" % (exitcode, cmd))
__main__.ExecutionError: The command return non-zero exitcode 1, cmd: 'sudo -E pkg install -y autoconf automake libtool openssl log4cplus boost-libs wget py38-sphinx py38-sphinx_rtd_theme'