Remove mention/use of Python 2 from discussion about types

[p-j-smith]

Changes made:

• Remove examples and discussion of Python 2 from ch00python/023types.ipynb

• Change discussion about how the behaviour of operators depends on the types of the operands

  • Previously the discussion was around the Python 2 behaviour of the division operator
  • Now the example is of the addition operator applied to ints vs floats

• Mention that division always returns a float. To perform integer division the divmod function can be used

• Added note about always using at least one digit after a decimal point to improve readability

[p-j-smith]

Closing as the actions can't be run from a fork - will open a new PR