Make python3 the default for `.py`

[RagnarGrootKoerkamp]

python 2 is dead now. Can we change the default language for the .py extension to python3?

Obviously this is not backwards compatible, but when needed it can easily be solved by putting #!/usr/bin/env python2 at the top of a python2 .py file.

For BAPCtools, I'm using the following in languages.yaml:

# Python2 with shebang comes before default python3.
python2_with_shebang:
    name: 'Python 2'
    priority: 860
    files: '*.py *.py2'
    shebang: '^#!.*python2\b'
    compile: '/usr/bin/python2 -m py_compile {files}'
    run: '/usr/bin/python2 {mainfile}'

python3:
    name: 'Python 3'
    priority: 850
    files: '*.py *.py3'
    compile: '/usr/bin/python3 -m py_compile {files}'
    run: '/usr/bin/python3 {mainfile}'

# Python2 without shebang comes after python3.
python2:
    name: 'Python 2'
    priority: 840
    files: '*.py2'
    compile: '/usr/bin/python2 -m py_compile {files}'
    run: '/usr/bin/python2 {mainfile}'

(side question: I don't think problemtools supports overriding languages.yaml by a repository/contest-level languages.yaml, right? Otherwise one could just add this snippet to the contest specific configuration.)

[mpsijm]

Something else I've just noticed: apparently, Kattis uses PyPy (https://open.kattis.com/help/python3), but Problemtools does not. I guess that should also be changed?