Maintenance: Remove trailing whitespaces, upgrade to Python 3 syntax, add pre-commit configuration

[EwoutH]

This PR does two maintenance things:

• It removes all trailing whitespaces
• It upgrades leftover Python 2 code to the Python 3 syntax

It also adds a pre-commit configuration, defined in .pre-commit-config.yaml, which checks for these things automatically. It checks once a month if there are any updates, so it's easy on maintanance.

pre-commit can be enabled on https://pre-commit.ci/.

There are a lot of other pre-commit hooks available that check automatically for things, see https://pre-commit.com/hooks.html.

[EwoutH]

@dhadka What do you think of this?

[dhadka]

It looks good to me. I'll follow up after merging to see if we can remove six.