Configsuite

**Miscellaneous**
- Stop running CI on Travis and move PyPI deploy to GitHub Actions

**New features**
- Add possibility in schema to specify whether a variable length container (list or dict) is allowed to be empty

**Miscellaneous**
- Replace usage of deprecated inspect methods

**Improvements**
- Validate that content is specified in schema if and only if element is a container.

**Deprecations**
- Drop Python 2.7 support

**Dependencies**
- Remove dependency on six

**Improvements**
- Have the Sphinx plugin only render examples if they are provided

**Bugfix**
- Make the Sphinx plugin's CSS changes local to the plugin

**Improvements**
- Better validation message for incompatible required value

**Bugfix**
- Pass on `deduce_required` when pushing

Bugfix: Default all containers to the empty container