Merge branch 'py3' into 'master'

Merge branch 'py3' into 'master'

Python 3

I tried to keep commits separate, so if anything causes trouble, it can be reverted or changed easily.

* pre-commit hooks pass
* all tests pass
* My use of `build`, `checkupdates`, `lint`, `import`, `publish` and `update` work as usual
* 2to3 does not report anything useful anymore (only useless parentheses and list() encapsulation of iterators)
* rewritemeta works exactly as usual

CC @eighthave

See merge request !88