Default Python files to Python 3

Currently, except if the file has a shebang pointing the pyhon3 binary, Python files are reported as text/x-python. Python 2 will reach end of support at the end of the year and hopefully most people are writing Python 3 code nowadays. I think it's time to consider defaulting to text/x-python3 if the version cannot be determined reliably.

What that would mean (and that's open to discussion):

• Files reported as text/x-python3:
  • *.py files with no shebang.
  • *.py files with shebang pointing python or python3.
• Files reported as text/x-python:
  • *.py files with shebang pointing python2.