Drop Python 2 support

[sphuber]

Fixes #514

The following commands were executed to clean all source files:

sed -i '/from __future__.*$/d' {} +
sed -i 's/(object):/:/g' {} +
sed -i 's/super([^)]*,[^)]*)/super()/g' {} +
sed -i '/\(import six\|from six\).*$/d' {} +
sed -i '/\(six\.add_metaclass\).*$/d' {} +
sed -i 's/six.integer_types/int/g' {} +
sed -i 's/six.string_types/str/g' {} +
sed -i 's/six.text_type/str/g' {} +
sed -i 's/six.binary_type/bytes/g' {} +
sed -i 's/six.moves.StringIO/io.StringIO/g' {} +
sed -i 's/six.StringIO/io.StringIO/g' {} +
sed -i 's/six.moves.BytesIO/io.BytesIO/g' {} +
sed -i 's/six.BytesIO/io.BytesIO/g' {} +
sed -i 's/six.moves.range/range/g' {} +
sed -i 's|six.iterkeys(\([^)]*\))|\1.keys()|' {} +
sed -i 's|six.iteritems(\([^)]*\))|\1.items()|' {} +
sed -i 's|six.itervalues(\([^)]*\))|\1.values()|' {} +

The operations were applied to all files using find -exec:

find . -type f -not -path './.git*' -exec

Note the exclusion of the .git folder which is very important.

[greschd]

Aw yeah 🥳 I've added a few minor comments - good to go for me once the tests pass.

[greschd]

I think we were missing the six requirement before 😅

[greschd]

It seems the pre-commit 2.X series supports only py36+

[sphuber]

It seems the pre-commit 2.X series supports only py36+

yeah, in my other projects I therefore split the dev extra into tests and pre-commit. That way I can just run pre-commit on CI with python 3.8 with latest packages and still run tests with python 3.5. For now I just downgraded pre-commit because I don't want to mix those extra changes in this huge PR. I will do that straight after this is merged with some additional CI related stuff

[greschd]

Yeah I think that's good practice anyway - the fewer extra dependencies are installed when testing, the less likely to forget something that should be in install_requires.

[greschd]

Might need a change in branch protection settings.