Fix for Python 4

[hugovk]

We don't yet know if 3.10 or 4.0 will follow Python 3.9, but whichever it is, it will probably happen in 2020 when Python 3.9 reaches beta and work begins on Python 3.9+1.

On Python 4, this would exit:

if sys.version_info[0] == 3:
    print("This script must be run with Python 3, exiting...")
    sys.exit(1)

Found using https://github.com/asottile/flake8-2020:

$ pip install -U flake8-2020
...
$ flake8 --select YTT
./tools/gen_latex_symbols.py:14:8: YTT201 `sys.version_info[0] == 3` referenced (python4), use `>=`

[Carreau]

Sounds great !